bpo-43795: PEP-652: Simplify headers for easier static analysis (GH-25483)

As part of the PEP-652 implementation, I'll tighten the CI check
for functions/data defined with `Py_LIMITED_API`.

Discussion in https://discuss.python.org/t/pep-652-maintaining-the-stable-abi/6986
suggests that parsing C headers is OK (though personally I'd rather generate it...),
but writing a full C parser is a monumental task and adding an existing one as a
dependency brings too many vendoring/bootstraping issues.

So, for the check I'll use a "simple" regex on preprocessor output, and adapt
the few trivial places where the regex won't work.

- Keep declarations in the limited API to one item per line
- Make it possible to override `_Py_NO_RETURN`, so the annotation can be
  removed from preprocessor output.


https://bugs.python.org/issue43795