gh-119993 ignore NotADirectoryError in Path.unlink() if missing_ok is True

[MusicalNinjaDad]

Add a short note on when Path.unlink() raises NotADirectoryError

This can occur "in the wild", for example, after calling shutil.move(src, dst) where dst is a non-existent directory. Debugging can cost a lot of time as the cause can be non-intuitive, potentially not nearby in the code and the docs don't mention the exception as being raised in any circumstances.
(This just happened to me after encountering pypa/cibuildwheel#1850)

Fixes: #119993

• Issue: pathlib.Path.unlink raises a NotADirectoryError if some element of the path is actually a file - this is undocumented #119993

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📚 Documentation preview 📚: https://cpython-previews--120049.org.readthedocs.build/

[MusicalNinjaDad]

@barneygale - this is how unlink would/could work if it were to ignore NotADirectoryError. Semantically that makes success with missing_ok = True mean "The file is not there afterwards" and with missing_ok = False to mean "The file was deleted". I think this makes sense, but I have no strong feelings that it must be like this.

Windows works differently from Linux/MacOS and raises FileNotFoundError if an intermediate path refers to a file. Without this change the same end-user code will potentially raise on one OS and not on another for the same situation.

[barneygale]

Add a @needs_posix decorator to the test and don't bother testing Windows IMO :]

[MusicalNinjaDad]

Does this hurt to have it in? Isn't it generally better to have the extra test coverage, or are we really struggling to keep execution times down?

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