Add license classifier back to pyproject.toml (#19599)

Co-authored-by: Alex Waygood <[email protected]>

Author: MichaReiser

pyproject.toml

@@ -23,6 +23,7 @@ classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",  # for compatibility with tooling such as pip-licenses
     "Operating System :: OS Independent",
     "Programming Language :: Python",
     "Programming Language :: Python :: 3.7",