GH-89812: Add pathlib._PurePathExt

[barneygale]

Move __fspath__(), __bytes__() and as_uri() methods from PurePath to a new _PurePathExt subclass. This new subclass is inherited by PurePosixPath, PureWindowsPath and Path.

Because PurePath isn't directly instantiatable (you get a PurePosixPath or PureWindowsPath instance back), this shouldn't change user-facing behaviour.

The methods must not be inherited future tarfile.TarPath and pathlib.AbstractPath classes, which will subclass PurePath.

• Discussion: https://discuss.python.org/t/make-pathlib-extensible/3428/94

• Issue: Add tarfile.TarPath #89812