GH-89812: Add `pathlib._PurePathExt` by barneygale · Pull Request #104810 · python/cpython

I've revised the implementation such that the methods are actually removed from PurePath. This will allow our future AbstractPath class to inherit PurePath, as it should conceptually:

(Contrast this with the earlier proposed hierarchy where PurePath and AbstractPath are siblings.

The methods are moved to a new _PurePathExt class, which is inherited by PurePosixPath, PureWindowsPath and Path.

This change will not be visible to users who try to instantiate PurePath, because they'll always get a PurePosixPath or PureWindowsPath instance back.

It will be visible to users who subclass PurePath, but that has only just become possible in 3.12. As such I think it's worth backporting this into 3.12 for consistency.