Coverage: exclude some noise

[brianschubert]

Going through the current test coverage results, there's quite a bit of noise from things that are not expected to be executed or would not be meaningful to test. For example, dummy functions with placeholder bodies:

typing_extensions/src/test_typing_extensions.py

Lines 7788 to 7789 in e589a26

	def __buffer__(self, flags: int) -> memoryview:
	return memoryview(b'')

and unexpected error handling:

typing_extensions/src/test_typing_extensions.py

Lines 5958 to 5965 in e589a26

	try:
	proc = subprocess.run(
	[sys.executable, "-c", code], check=True, capture_output=True, text=True,
	)
	except subprocess.CalledProcessError as exc:
	print("stdout", exc.stdout, sep="\n")
	print("stderr", exc.stderr, sep="\n")
	raise

It would be nice to exclude these from the coverage report so that it's easier to see what's actually missing coverage.

This PR adds exclusions for:

• pass and raise NotImplementedError used as placeholder bodies (via pyproject.toml config)
• various ad-hoc placeholder bodies in the tests (via # pragma: no cover comments)
  • (another option would be to normalize these bodies to some common form that can be more easily excluded en-masse, like raise NotImplementedError or ...)
• two other cases which don't seem meaningful to test, namely the if __name__ == '__main__' block in the tests and the __repr__ of an internal sentinel

[brianschubert]

Before: https://github.com/python/typing_extensions/actions/runs/17179580699

After: https://github.com/python/typing_extensions/actions/runs/17191135859?pr=656

[JelleZijlstra]

This is fine but maybe we should eat our dogfood and use typing_extensions.Sentinel.

[AlexWaygood]

Or if not, we could also just add a test for this. I know it's trivial code, but it doesn't really do any harm to test it explicitly, and it's nice to keep pragma: no cover exclusions to a minimum.

[brianschubert]

Dogfooding Sentinel sounds neat - I opened #657 to give that a try