gh-119671: Add os.path.pure

[nineteendo]

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📚 Documentation preview 📚: https://cpython-previews--119772.org.readthedocs.build/

• Issue: Some ntpath/posixpath functions unusable on foreign OS #119671

[nineteendo]

@barneygale, it could look something like this.

[nineteendo]

@barneygale, all tests are passing now. I made os.path.pure simply export all safe functions from os.path.
When this is merged, the safe functions could be moved from os.path to os.path.pure.

Should defpath & devnull be exported?

[nineteendo]

Thanks Erlend.