GH-104898: Revert pathlib os.PathLike registration change.

[barneygale]

Subclassing os.PathLike rather than using register() makes initialisation slower, due to the additional __isinstance__ work.

This partially reverts commit bd1b622.

$ ./python -m timeit -s 'from pathlib import PurePath' 'PurePath("a", "b", "c")'
100000 loops, best of 5: 2.12 usec per loop # before
200000 loops, best of 5: 1.69 usec per loop # after

• Issue: os.PathLike is missing __slots__ #104898

[merwok]

Note that you can use ABC.register as a class decorator, but I think it would not look better for this case.