diff --git a/.gitignore b/.gitignore
index 2db46b823..83ff4ab4c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -105,3 +105,5 @@ env3?/
 
 # MyPy cache
 .mypy_cache/
+
+all_known_setup.yaml
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 623e5d605..71f9e72b4 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -24,12 +24,16 @@ repos:
     args: ["--line-length=120"]
 
 - repo: https://github.com/pre-commit/mirrors-mypy
-  rev: v0.790
+  rev: v0.800
   hooks:
   - id: mypy
-    files: ^(cibuildwheel/|test/|bin/projects.py|bin/update_pythons.py|unit_test/)
-    pass_filenames: false
-    additional_dependencies: [packaging, click]
+    exclude: ^(bin/|cibuildwheel/resources/).*py$
+    additional_dependencies: [packaging]
+  - id: mypy
+    name: mypy 3.7+ on bin/
+    files: ^bin/.*py$
+    additional_dependencies: [packaging]
+    args: ["--python-version=3.7", "--ignore-missing-imports", "--scripts-are-modules"]
 
 - repo: https://github.com/asottile/pyupgrade
   rev: v2.7.4
diff --git a/README.md b/README.md
index af701624b..841b4f3ac 100644
--- a/README.md
+++ b/README.md
@@ -103,6 +103,7 @@ Options
 | **Build selection** | [`CIBW_PLATFORM`](https://cibuildwheel.readthedocs.io/en/stable/options/#platform)  | Override the auto-detected target platform |
 |   | [`CIBW_BUILD`](https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip)  <br> [`CIBW_SKIP`](https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip)  | Choose the Python versions to build |
 |   | [`CIBW_ARCHS_LINUX`](https://cibuildwheel.readthedocs.io/en/stable/options/#archs)  | Build non-native architectures |
+|   | [`CIBW_PROJECT_REQUIRES_PYTHON`](https://cibuildwheel.readthedocs.io/en/stable/options/#requires-python)  | Manually set the Python compatibility of your project |
 | **Build customization** | [`CIBW_ENVIRONMENT`](https://cibuildwheel.readthedocs.io/en/stable/options/#environment)  | Set environment variables needed during the build |
 |   | [`CIBW_BEFORE_ALL`](https://cibuildwheel.readthedocs.io/en/stable/options/#before-all)  | Execute a shell command on the build system before any wheels are built. |
 |   | [`CIBW_BEFORE_BUILD`](https://cibuildwheel.readthedocs.io/en/stable/options/#before-build)  | Execute a shell command preparing each wheel's build |
diff --git a/bin/inspect_all_known_projects.py b/bin/inspect_all_known_projects.py
new file mode 100755
index 000000000..5c6547358
--- /dev/null
+++ b/bin/inspect_all_known_projects.py
@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+from __future__ import annotations
+
+import ast
+from pathlib import Path
+from typing import Iterator, Optional
+
+import click
+import yaml
+from ghapi.core import GhApi, HTTP404NotFoundError  # type: ignore
+from rich import print
+
+from cibuildwheel.projectfiles import Analyzer
+
+DIR = Path(__file__).parent.resolve()
+
+
+def parse(contents: str) -> Optional[str]:
+    try:
+        tree = ast.parse(contents)
+        analyzer = Analyzer()
+        analyzer.visit(tree)
+        return analyzer.requires_python or ""
+    except Exception:
+        return None
+
+
+def check_repo(name: str, contents: str) -> str:
+    s = f"  {name}: "
+    if name == "setup.py":
+        if "python_requires" not in contents:
+            s += "❌"
+        res = parse(contents)
+        if res is None:
+            s += "⚠️ "
+        elif res:
+            s += "✅ " + res
+        elif "python_requires" in contents:
+            s += "☑️"
+
+    elif name == "setup.cfg":
+        s += "✅" if "python_requires" in contents else "❌"
+    else:
+        s += "✅" if "requires-python" in contents else "❌"
+
+    return s
+
+
+class MaybeRemote:
+    def __init__(self, cached_file: Path | str, *, online: bool) -> None:
+        self.online = online
+        if self.online:
+            self.contents: dict[str, dict[str, Optional[str]]] = {
+                "setup.py": {},
+                "setup.cfg": {},
+                "pyproject.toml": {},
+            }
+        else:
+            with open(cached_file) as f:
+                self.contents = yaml.safe_load(f)
+
+    def get(self, repo: str, filename: str) -> Optional[str]:
+        if self.online:
+            try:
+                self.contents[filename][repo] = (
+                    GhApi(*repo.split("/")).get_content(filename).decode()
+                )
+            except HTTP404NotFoundError:
+                self.contents[filename][repo] = None
+            return self.contents[filename][repo]
+        elif repo in self.contents[filename]:
+            return self.contents[filename][repo]
+        else:
+            raise RuntimeError(
+                f"Trying to access {repo}:{filename} and not in cache, rebuild cache"
+            )
+
+    def save(self, filename: Path | str) -> None:
+        with open(filename, "w") as f:
+            yaml.safe_dump(self.contents, f, default_flow_style=False)
+
+    def on_each(self, repos: list[str]) -> Iterator[tuple[str, str, Optional[str]]]:
+        for repo in repos:
+            print(f"[bold]{repo}:")
+            for filename in sorted(self.contents, reverse=True):
+                yield repo, filename, self.get(repo, filename)
+
+
+@click.command()
+@click.option("--online", is_flag=True, help="Remember to set GITHUB_TOKEN")
+def main(online: bool) -> None:
+    with open(DIR / "../docs/data/projects.yml") as f:
+        known = yaml.safe_load(f)
+
+    repos = [x["gh"] for x in known]
+
+    ghinfo = MaybeRemote("all_known_setup.yaml", online=online)
+
+    for _, filename, contents in ghinfo.on_each(repos):
+        if contents is None:
+            print(f"[red]  {filename}: Not found")
+        else:
+            print(check_repo(filename, contents))
+
+    if online:
+        ghinfo.save("all_known_setup.yaml")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/cibuildwheel/__main__.py b/cibuildwheel/__main__.py
index 3b7e2af75..33a570c7f 100644
--- a/cibuildwheel/__main__.py
+++ b/cibuildwheel/__main__.py
@@ -7,12 +7,15 @@
 from pathlib import Path
 from typing import Dict, List, Optional, Set, Union, overload
 
+from packaging.specifiers import SpecifierSet
+
 import cibuildwheel
 import cibuildwheel.linux
 import cibuildwheel.macos
 import cibuildwheel.windows
 from cibuildwheel.architecture import Architecture, allowed_architectures_check
 from cibuildwheel.environment import EnvironmentParseError, parse_environment
+from cibuildwheel.projectfiles import get_requires_python_str
 from cibuildwheel.typing import PLATFORMS, PlatformName, assert_never
 from cibuildwheel.util import (
     BuildOptions,
@@ -26,10 +29,10 @@
 
 
 @overload
-def get_option_from_environment(option_name: str, platform: Optional[str], default: str) -> str: ...  # noqa: E704
+def get_option_from_environment(option_name: str, *, platform: Optional[str] = None, default: str) -> str: ...  # noqa: E704
 @overload
-def get_option_from_environment(option_name: str, platform: Optional[str] = None, default: None = None) -> Optional[str]: ...  # noqa: E704 E302
-def get_option_from_environment(option_name: str, platform: Optional[str] = None, default: Optional[str] = None) -> Optional[str]:  # noqa: E302
+def get_option_from_environment(option_name: str, *, platform: Optional[str] = None, default: None = None) -> Optional[str]: ...  # noqa: E704 E302
+def get_option_from_environment(option_name: str, *, platform: Optional[str] = None, default: Optional[str] = None) -> Optional[str]:  # noqa: E302
     '''
     Returns an option from the environment, optionally scoped by the platform.
 
@@ -156,7 +159,18 @@ def main() -> None:
     test_extras = get_option_from_environment('CIBW_TEST_EXTRAS', platform=platform, default='')
     build_verbosity_str = get_option_from_environment('CIBW_BUILD_VERBOSITY', platform=platform, default='')
 
-    build_selector = BuildSelector(build_config=build_config, skip_config=skip_config)
+    package_files = {'setup.py', 'setup.cfg', 'pyproject.toml'}
+
+    if not any(package_dir.joinpath(name).exists() for name in package_files):
+        names = ', '.join(sorted(package_files, reverse=True))
+        print(f'cibuildwheel: Could not find any of {{{names}}} at root of package', file=sys.stderr)
+        sys.exit(2)
+
+    # Passing this in as an environment variable will override pyproject.toml, setup.cfg, or setup.py
+    requires_python_str: Optional[str] = os.environ.get('CIBW_PROJECT_REQUIRES_PYTHON') or get_requires_python_str(package_dir)
+    requires_python = None if requires_python_str is None else SpecifierSet(requires_python_str)
+
+    build_selector = BuildSelector(build_config=build_config, skip_config=skip_config, requires_python=requires_python)
     test_selector = TestSelector(skip_config=test_skip)
 
     try:
@@ -186,15 +200,11 @@ def main() -> None:
     # This needs to be passed on to the docker container in linux.py
     os.environ['CIBUILDWHEEL'] = '1'
 
-    if not any((package_dir / name).exists()
-               for name in ["setup.py", "setup.cfg", "pyproject.toml"]):
-        print('cibuildwheel: Could not find setup.py, setup.cfg or pyproject.toml at root of package', file=sys.stderr)
-        sys.exit(2)
-
     if args.archs is not None:
         archs_config_str = args.archs
     else:
         archs_config_str = get_option_from_environment('CIBW_ARCHS', platform=platform, default='auto')
+
     archs = Architecture.parse_config(archs_config_str, platform=platform)
 
     identifiers = get_build_identifiers(platform, build_selector, archs)
@@ -334,6 +344,7 @@ def get_build_identifiers(
     python_configurations: Union[List[cibuildwheel.linux.PythonConfiguration],
                                  List[cibuildwheel.windows.PythonConfiguration],
                                  List[cibuildwheel.macos.PythonConfiguration]]
+
     if platform == 'linux':
         python_configurations = cibuildwheel.linux.get_python_configurations(build_selector, architectures)
     elif platform == 'windows':
diff --git a/cibuildwheel/projectfiles.py b/cibuildwheel/projectfiles.py
new file mode 100644
index 000000000..8d49fe4d6
--- /dev/null
+++ b/cibuildwheel/projectfiles.py
@@ -0,0 +1,79 @@
+import ast
+import sys
+from configparser import ConfigParser
+from pathlib import Path
+from typing import Any, Optional
+
+import toml
+
+if sys.version_info < (3, 8):
+    Constant = ast.Str
+
+    def get_constant(x: ast.Str) -> str:
+        return x.s
+
+
+else:
+    Constant = ast.Constant
+
+    def get_constant(x: ast.Constant) -> Any:
+        return x.value
+
+
+class Analyzer(ast.NodeVisitor):
+    def __init__(self) -> None:
+        self.requires_python: Optional[str] = None
+
+    def visit(self, content: ast.AST) -> None:
+        for node in ast.walk(content):
+            for child in ast.iter_child_nodes(node):
+                child.parent = node  # type: ignore
+        super().visit(content)
+
+    def visit_keyword(self, node: ast.keyword) -> None:
+        self.generic_visit(node)
+        if node.arg == "python_requires":
+            # Must not be nested in an if or other structure
+            # This will be Module -> Expr -> Call -> keyword
+            if (
+                not hasattr(node.parent.parent.parent, "parent")  # type: ignore
+                and isinstance(node.value, Constant)
+            ):
+                self.requires_python = get_constant(node.value)
+
+
+def setup_py_python_requires(content: str) -> Optional[str]:
+    try:
+        tree = ast.parse(content)
+        analyzer = Analyzer()
+        analyzer.visit(tree)
+        return analyzer.requires_python or None
+    except Exception:
+        return None
+
+
+def get_requires_python_str(package_dir: Path) -> Optional[str]:
+    "Return the python requires string from the most canonical source available, or None"
+
+    # Read in from pyproject.toml:project.requires-python
+    try:
+        info = toml.load(package_dir / 'pyproject.toml')
+        return str(info['project']['requires-python'])
+    except (FileNotFoundError, KeyError, IndexError, TypeError):
+        pass
+
+    # Read in from setup.cfg:options.python_requires
+    try:
+        config = ConfigParser()
+        config.read(package_dir / 'setup.cfg')
+        return str(config['options']['python_requires'])
+    except (FileNotFoundError, KeyError, IndexError, TypeError):
+        pass
+
+    try:
+        with open(package_dir / 'setup.py') as f:
+            return setup_py_python_requires(f.read())
+    except FileNotFoundError:
+        pass
+
+    return None
diff --git a/cibuildwheel/util.py b/cibuildwheel/util.py
index 9e0bc5d82..761a964ba 100644
--- a/cibuildwheel/util.py
+++ b/cibuildwheel/util.py
@@ -13,12 +13,15 @@
 import bracex
 import certifi
 import toml
+from packaging.specifiers import SpecifierSet
+from packaging.version import Version
 
 from .architecture import Architecture
 from .environment import ParsedEnvironment
 from .typing import PathOrStr, PlatformName
 
 resources_dir = Path(__file__).parent / 'resources'
+
 get_pip_script = resources_dir / 'get-pip.py'
 install_certifi_script = resources_dir / "install_certifi.py"
 
@@ -53,13 +56,25 @@ class IdentifierSelector:
     """
     This class holds a set of build/skip patterns. You call an instance with a
     build identifier, and it returns True if that identifier should be
-    included.
+    included. Only call this on valid identifiers, ones that have at least 2
+    numeric digits before the first dash.
     """
-    def __init__(self, *, build_config: str, skip_config: str):
+
+    def __init__(self, *, build_config: str, skip_config: str, requires_python: Optional[SpecifierSet] = None):
         self.build_patterns = build_config.split()
         self.skip_patterns = skip_config.split()
+        self.requires_python = requires_python
 
     def __call__(self, build_id: str) -> bool:
+        # Filter build selectors by python_requires if set
+        if self.requires_python is not None:
+            py_ver_str = build_id.split('-')[0]
+            major = int(py_ver_str[2])
+            minor = int(py_ver_str[3:])
+            version = Version(f"{major}.{minor}.99")
+            if not self.requires_python.contains(version):
+                return False
+
         build_patterns = itertools.chain.from_iterable(bracex.expand(p) for p in self.build_patterns)
         skip_patterns = itertools.chain.from_iterable(bracex.expand(p) for p in self.skip_patterns)
 
@@ -78,6 +93,8 @@ class BuildSelector(IdentifierSelector):
     pass
 
 
+# Note that requires-python is not needed for TestSelector, as you can't test
+# what you can't build.
 class TestSelector(IdentifierSelector):
     def __init__(self, *, skip_config: str):
         super().__init__(build_config="*", skip_config=skip_config)
diff --git a/docs/options.md b/docs/options.md
index dc46a1108..78ffce929 100644
--- a/docs/options.md
+++ b/docs/options.md
@@ -208,6 +208,61 @@ Platform-specific variants also available:<br/>
 
 This option can also be set using the [command-line option](#command-line) `--archs`.
 
+###  `CIBW_PROJECT_REQUIRES_PYTHON` {: #requires-python}
+> Manually set the Python compatibility of your project
+
+By default, cibuildwheel reads your package's Python compatibility from
+`pyproject.toml` following [PEP621](https://www.python.org/dev/peps/pep-0621/)
+or from `setup.cfg`; finally it will try to inspect the AST of `setup.py` for a
+simple keyword assignment in a top level function call. If you need to override
+this behaviour for some reason, you can use this option.
+
+When setting this option, the syntax is the same as `project.requires-python`,
+using 'version specifiers' like `>=3.6`, according to
+[PEP440](https://www.python.org/dev/peps/pep-0440/#version-specifiers).
+
+Default: reads your package's Python compatibility from `pyproject.toml`
+(`project.requires-python`) or `setup.cfg` (`options.python_requires`) or
+`setup.py` `setup(python_requires="...")`. If not found, cibuildwheel assumes
+the package is compatible with all versions of Python that it can build.
+
+
+!!! note
+    Rather than using this option, it's recommended you set
+    `project.requires-python` in `pyproject.toml` instead:
+    Example `pyproject.toml`:
+
+        [project]
+        requires-python = ">=3.6"
+
+        # Aside - in pyproject.toml you should always specify minimal build
+        # system options, like this:
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
+
+    Currently, setuptools has not yet added support for reading this value from
+    pyproject.toml yet, and so does not copy it to Requires-Python in the wheel
+    metadata. This mechanism is used by `pip` to scan through older versions of
+    your package until it finds a release compatible with the curernt version
+    of Python compatible when installing, so it is an important value to set if
+    you plan to drop support for a version of Python in the future.
+
+    If you don't want to list this value twice, you can also use the setuptools
+    specific location in `setup.cfg` and cibuildwheel will detect it from
+    there. Example `setup.cfg`:
+
+        [options]
+        python_requires = ">=3.6"
+
+#### Examples
+
+```yaml
+CIBW_PROJECT_REQUIRES_PYTHON: ">=3.6"
+```
+
 ## Build customization
 
 ### `CIBW_ENVIRONMENT` {: #environment}
diff --git a/setup.cfg b/setup.cfg
index 251f0527e..a8815a044 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -32,6 +32,7 @@ install_requires =
     toml
     certifi
     bracex
+    packaging
     typing_extensions; python_version < '3.8'
 
 [options.package_data]
@@ -42,17 +43,17 @@ dev =
     click
     mkdocs-include-markdown-plugin==2.1.1
     mkdocs==1.0.4
-    mypy
     pip-tools
     pygithub
+    ghapi
     pymdown-extensions
-    pytest
+    pytest>=4
     pytest-timeout
     pyyaml
     requests
     typing-extensions
-    packaging>=20.8
     rich>=9.6
+    mypy>=0.800
 
 [options.packages.find]
 include =
@@ -83,14 +84,14 @@ junit_family=xunit2
 
 [mypy]
 python_version = 3.6
-files = cibuildwheel,test,unit_test,bin
+files = cibuildwheel/*.py,test/**/*.py,unit_test/**/*.py
 warn_unused_configs = True
 warn_redundant_casts = True
 
-[mypy-test.*]
+[mypy-test.*,mypy-unit_test.*]
 check_untyped_defs = True
 
-[mypy-cibuildwheel.*,unit_test.*]
+[mypy-cibuildwheel.*]
 disallow_any_generics = True
 disallow_subclassing_any = True
 disallow_untyped_calls = True
@@ -122,6 +123,9 @@ ignore_missing_imports = True
 [mypy-bracex.*]
 ignore_missing_imports = True
 
+[mypy-importlib_resources.*]
+ignore_missing_imports = True
+
 [tool:isort]
 profile=black
 multi_line_output=3
diff --git a/unit_test/__init__.py b/unit_test/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/unit_test/build_ids_test.py b/unit_test/build_ids_test.py
index 1993efd03..b0d4188a1 100644
--- a/unit_test/build_ids_test.py
+++ b/unit_test/build_ids_test.py
@@ -1,11 +1,8 @@
-import pytest
 import toml
-
-from cibuildwheel.util import resources_dir
-
-Version = pytest.importorskip("packaging.version").Version
+from packaging.version import Version
 
 from cibuildwheel.extra import InlineArrayDictEncoder  # noqa: E402
+from cibuildwheel.util import resources_dir
 
 
 def test_compare_configs():
diff --git a/unit_test/build_selector_test.py b/unit_test/build_selector_test.py
index 6e00fc769..227636bcc 100644
--- a/unit_test/build_selector_test.py
+++ b/unit_test/build_selector_test.py
@@ -1,3 +1,5 @@
+from packaging.specifiers import SpecifierSet
+
 from cibuildwheel.util import BuildSelector
 
 
@@ -69,3 +71,36 @@ def test_build_braces():
     assert build_selector('cp37-manylinux1_x86_64')
     assert not build_selector('cp38-manylinux1_x86_64')
     assert not build_selector('cp39-manylinux1_x86_64')
+
+
+def test_build_limited_python():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=3.6"))
+
+    assert not build_selector('cp27-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+    assert build_selector('cp37-manylinux1_x86_64')
+    assert not build_selector('cp27-manylinux1_i686')
+    assert build_selector('cp36-manylinux1_i686')
+    assert build_selector('cp37-manylinux1_i686')
+    assert not build_selector('cp27-win32')
+    assert build_selector('cp36-win32')
+    assert build_selector('cp37-win32')
+    assert not build_selector('pp27-win32')
+    assert build_selector('pp36-win32')
+    assert build_selector('pp37-win32')
+
+
+def test_build_limited_python_partial():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*"))
+
+    assert build_selector('cp27-manylinux1_x86_64')
+    assert not build_selector('cp35-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+
+
+def test_build_limited_python_patch():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=2.7.9"))
+
+    assert build_selector('cp27-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+    assert build_selector('cp37-manylinux1_x86_64')
diff --git a/unit_test/main_tests/__init__.py b/unit_test/main_tests/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/unit_test/main_tests/main_options_test.py b/unit_test/main_tests/main_options_test.py
index b55a7db6c..9342dca82 100644
--- a/unit_test/main_tests/main_options_test.py
+++ b/unit_test/main_tests/main_options_test.py
@@ -51,8 +51,8 @@ def test_build_selector(platform, intercepted_build_args, monkeypatch, allow_emp
 
     intercepted_build_selector = intercepted_build_args.args[0].build_selector
     assert isinstance(intercepted_build_selector, BuildSelector)
-    assert intercepted_build_selector('build-this')
-    assert not intercepted_build_selector('skip-that')
+    assert intercepted_build_selector('build24-this')
+    assert not intercepted_build_selector('skip65-that')
     # This unit test is just testing the options of 'main'
     # Unit tests for BuildSelector are in build_selector_test.py
 
diff --git a/unit_test/main_tests/main_platform_test.py b/unit_test/main_tests/main_platform_test.py
index f86de5409..98bcfcdd9 100644
--- a/unit_test/main_tests/main_platform_test.py
+++ b/unit_test/main_tests/main_platform_test.py
@@ -1,10 +1,11 @@
 import sys
 
 import pytest
-from conftest import MOCK_PACKAGE_DIR  # noqa: I100
 
 from cibuildwheel.__main__ import main
-from cibuildwheel.util import Architecture
+from cibuildwheel.architecture import Architecture
+
+from .conftest import MOCK_PACKAGE_DIR
 
 
 def test_unknown_platform_non_ci(monkeypatch, capsys):
diff --git a/unit_test/main_tests/main_requires_python_test.py b/unit_test/main_tests/main_requires_python_test.py
new file mode 100644
index 000000000..a9c86bc2d
--- /dev/null
+++ b/unit_test/main_tests/main_requires_python_test.py
@@ -0,0 +1,103 @@
+import sys
+import textwrap
+
+import pytest
+from packaging.specifiers import SpecifierSet
+
+from cibuildwheel.__main__ import main
+
+
+@pytest.fixture(autouse=True, scope="function")
+def fake_package_dir(monkeypatch, tmp_path):
+    '''
+    Set up a fake project
+    '''
+
+    local_path = tmp_path / "tmp_project"
+    local_path.mkdir()
+
+    local_path.joinpath("setup.py").touch()
+
+    monkeypatch.setattr(sys, 'argv', ['cibuildwheel', str(local_path)])
+
+    return local_path
+
+
+def test_no_override(platform, monkeypatch, intercepted_build_args):
+
+    main()
+
+    intercepted_build_selector = intercepted_build_args.args[0].build_selector
+
+    assert intercepted_build_selector('cp39-win32')
+    assert intercepted_build_selector('cp36-win32')
+
+    assert intercepted_build_selector.requires_python is None
+
+
+def test_override_env(platform, monkeypatch, intercepted_build_args):
+    monkeypatch.setenv('CIBW_PROJECT_REQUIRES_PYTHON', '>=3.8')
+
+    main()
+
+    intercepted_build_selector = intercepted_build_args.args[0].build_selector
+
+    assert intercepted_build_selector.requires_python == SpecifierSet(">=3.8")
+
+    assert intercepted_build_selector('cp39-win32')
+    assert not intercepted_build_selector('cp36-win32')
+
+
+def test_override_setup_cfg(platform, monkeypatch, intercepted_build_args, fake_package_dir):
+
+    fake_package_dir.joinpath("setup.cfg").write_text(textwrap.dedent("""
+        [options]
+        python_requires = >=3.8
+    """))
+
+    main()
+
+    intercepted_build_selector = intercepted_build_args.args[0].build_selector
+
+    assert intercepted_build_selector.requires_python == SpecifierSet(">=3.8")
+
+    assert intercepted_build_selector('cp39-win32')
+    assert not intercepted_build_selector('cp36-win32')
+
+
+def test_override_pyproject_toml(platform, monkeypatch, intercepted_build_args, fake_package_dir):
+
+    fake_package_dir.joinpath("pyproject.toml").write_text(textwrap.dedent("""
+        [project]
+        requires-python = ">=3.8"
+    """))
+
+    main()
+
+    intercepted_build_selector = intercepted_build_args.args[0].build_selector
+
+    assert intercepted_build_selector.requires_python == SpecifierSet(">=3.8")
+
+    assert intercepted_build_selector('cp39-win32')
+    assert not intercepted_build_selector('cp36-win32')
+
+
+def test_override_setup_py_simple(platform, monkeypatch, intercepted_build_args, fake_package_dir):
+
+    fake_package_dir.joinpath("setup.py").write_text(textwrap.dedent("""
+        from setuptools import setup
+
+        setup(
+            name = "other",
+            python_requires = ">=3.7",
+        )
+    """))
+
+    main()
+
+    intercepted_build_selector = intercepted_build_args.args[0].build_selector
+
+    assert intercepted_build_selector.requires_python == SpecifierSet(">=3.7")
+
+    assert intercepted_build_selector('cp39-win32')
+    assert not intercepted_build_selector('cp36-win32')
diff --git a/unit_test/projectfiles_test.py b/unit_test/projectfiles_test.py
new file mode 100644
index 000000000..6b80a13ac
--- /dev/null
+++ b/unit_test/projectfiles_test.py
@@ -0,0 +1,142 @@
+from textwrap import dedent
+
+from cibuildwheel.projectfiles import get_requires_python_str, setup_py_python_requires
+
+
+def test_read_setup_py_simple(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = "1.23",
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) == "1.23"
+    assert get_requires_python_str(tmp_path) == "1.23"
+
+
+def test_read_setup_py_full(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            import setuptools
+
+            setuptools.randomfunc()
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = "1.24",
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) == "1.24"
+    assert get_requires_python_str(tmp_path) == "1.24"
+
+
+def test_read_setup_py_assign(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = "3.21"
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = REQUIRES,
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_py_None(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = None
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = None,
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_py_empty(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = "3.21"
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_cfg(tmp_path):
+    with open(tmp_path / "setup.cfg", "w") as f:
+        f.write(dedent("""
+            [options]
+            python_requires = 1.234
+            [metadata]
+            something = other
+            """))
+
+    assert get_requires_python_str(tmp_path) == "1.234"
+
+
+def test_read_setup_cfg_empty(tmp_path):
+    with open(tmp_path / "setup.cfg", "w") as f:
+        f.write(dedent("""
+            [options]
+            other = 1.234
+            [metadata]
+            something = other
+            """))
+
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_pyproject_toml(tmp_path):
+    with open(tmp_path / "pyproject.toml", "w") as f:
+        f.write(dedent("""
+            [project]
+            requires-python = "1.654"
+
+            [tool.cibuildwheel]
+            something = "other"
+            """))
+
+    assert get_requires_python_str(tmp_path) == "1.654"
+
+
+def test_read_pyproject_toml_empty(tmp_path):
+    with open(tmp_path / "pyproject.toml", "w") as f:
+        f.write(dedent("""
+            [project]
+            other = 1.234
+            """))
+
+    assert get_requires_python_str(tmp_path) is None