fix: tolerate None __file__ in _distutils_hack frame detector

[wahajahmed010]

Summary

_distutils_hack.DistutilsMetaFinder.frame_file_is_setup can crash during pip startup on Python 3.14+ when a frame's f_globals carries a __file__ set to None. The previous implementation used frame.f_globals.get('__file__', '').endswith('setup.py'), which only handles the missing-key case; when the key is present and the value is None, the .endswith call raises AttributeError: 'NoneType' object has no attribute 'endswith'. This is the same root cause behind the report in #5263: with a legacy setuptools installed, pip itself becomes unusable because the shim can't be imported.

The fix treats None and other non-string values the same as a missing __file__ (i.e. "not a setup.py frame"). The frame walker in pip_imported_during_build therefore completes without raising, and the rest of _distutils_hack continues to work as before.

How to reproduce (without my fix)

class Frame:
    @property
    def f_globals(self):
        return {"__file__": None}

# On current main:
from _distutils_hack import DistutilsMetaFinder
DistutilsMetaFinder.frame_file_is_setup(Frame())
# AttributeError: 'NoneType' object has no attribute 'endswith'

With this branch loaded instead, the same call returns False cleanly.

Pull Request Checklist

• Changes have tests (added test_frame_file_is_setup_handles_missing_file in setuptools/tests/test_distutils_adoption.py, covering missing key, None, a real setup.py path, and a non-matching path)
• News fragment added in newsfragments/5263.bugfix.rst

Closes #5263

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[abravalheri]

Hi @wahajahmed010 thank you very much for the contribution.

Would you be able to provide a real world reproducer that does not rely on a mock/stub for us to assess this issue?

You mentioned changes on Python 3.14+... Do you have a reference for that? I checked the release notes and I couldn't find any specific changes in __file__ that allow it to be None, the docs still say it has to be a string when present.