GH-76846, GH-85281: Call `__new__()` and `__init__()` on pathlib subclasses (GH-102789)

Fix an issue where `__new__()` and `__init__()` were not called on subclasses of `pathlib.PurePath` and `Path` in some circumstances.

Paths are now normalized on-demand. This speeds up path construction, `p.joinpath(q)`, and `p / q`.

Co-authored-by: Steve Dower <steve.dower@microsoft.com>