Run the CPython typing test suite against typing-extensions

[JelleZijlstra]

I propose to add a workflow that:

• Grabs test_typing.py from CPython
• Replaces all typing imports with typing_extensions through some hackery (either sed or some cleverness with importlib)
• Runs the test suite

This would help us catch issues where something got changed in CPython and we didn't reapply it to our variant. For example, this would allow us to catch issues in our TypedDict implementation, which is currently different from CPython's on all versions.

We'd have to run this only on matching versions (e.g., run the CPython tests from the 3.14 branch on Python 3.14) because the CPython tests may rely on new syntax or standard library features.

Feel free to pick this up if you see this and are interested! I may do it eventually when I find time.

[AlexWaygood]

Note that we already run test_typing after typing_extensions has been installed, but that's not as comprehensive as what's being proposed here:

typing_extensions/.github/workflows/ci.yml

Lines 74 to 81 in 44de568

	- name: Test CPython typing test suite
	# Test suite fails on PyPy even without typing_extensions
	if: ${{ !startsWith(matrix.python-version, 'pypy') }}
	run: |
	cd src
	# Run the typing test suite from CPython with typing_extensions installed,
	# because we monkeypatch typing under some circumstances.
	python -c 'import typing_extensions; import test.__main__' test_typing -v

[JelleZijlstra]

Right, that just tests that the monkeypatches we do to typing don't break things.