PythonDeps: move cli_only handling logic to _parse_requirements

[masenf]

• Remove cli_only parameter from build_parser.
• Remove special case handling for --hash option (only valid in requirements.txt files, not pip install).
• Validate options in PythonDeps._parse_requirements:
  • Only check cli_only logic for ParsedRequirement objects that directly come from the PythonDeps.
  • Allow included requirements.txt lines to correctly parse --hash (Fix tox4: fails to process requirement files with --hash #2373).
  • Provides a more contextual error message to end users when --hash is used in the deps list.

#2373 [test case] tox4: fails to process requirement files with --hash

• Specifying --hash in the deps list doesn't work (pip would reject this anyway).
• Specifying --hash in a requirements.txt file named in the deps list should work, and recursive parsing should correctly extract the hash.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword in message body
• added news fragment in changelog folder: 2373.bugfix.rst
• added yourself to CONTRIBUTORS (preserving alphabetical order)

Background

I just released a new plugin today, tox-pin-deps, and hit #2373 while trying to get it working on tox4.