Optimize pathlib path construction

[barneygale]

Pathlib is slow. One of the most obvious symptoms is that pathlib.PurePath objects are slow to construct. We should be able to speed construction up without making other parts of pathlib slower.

Two possible approaches:

1. Optimize the existing machinary of path construction: __new__(), _from_parts(), _parse_parts(), _parse_args().
2. Perform less work in the constructor: defer parsing, joining and normalization until needed.

Linked PRs

• GH-101362: Optimise pathlib by deferring path normalisation #101560
• GH-101362: Check pathlib.Path flavour compatibility at import time #101664
• GH-101362: Call join() only when >1 argument supplied to pathlib.PurePath() #101665
• GH-101362: Optimise PurePath(PurePath(...)) #101667
• GH-101362: Omit path anchor from pathlib.PurePath()._parts #102476

[barneygale]

I'd like to land #101363 before I put the first PR up for this issue.

[zooba]

Does the PR cope with WindowsPurePath and PosixPurePath and converting between them?

[barneygale]

Could you clarify? The PRs maintain the behaviour that attempting to instantiate pathlib.PurePath will give you a PureWindowsPath or a PurePosixPath depending on your platform, but pathlib doesn't support converting between Windows and POSIX paths as I understand it.