GH-104898: Revert pathlib os.PathLike registration change. (GH-105073)

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

This partially reverts commit bd1b6228d132b8e9836fe352cd8dca2b6c1bd98c.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>