Configurable Parser

[hasier]

Adds a class-variable to Parser so that subclasses can override which keywords the parser will recognise.
Similarly, adds a class-variable to Environment so that subclasses can override which Parser they'll use.

• fixes Configurable Parser #1850

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[hasier]

The test failing in 3.12-dev seems to also be failing in main, so it does not seem related to the changes here. Same issue in other repos:

• Deprecation warnings in Python 3.12a7 for usage of shutil.rmtree pypa/pip#11957
• Handle Python3.12a7 compatibility problems pypa/setuptools#3897