Optimize joining of pathlib.PurePath() arguments.

[barneygale]

In Python 3.12, when multiple arguments are given to PurePath(), the initialiser calls os.path.join() to join them. This is reasonably slow. For Python 3.13 we can make it faster by:

1. Deferring joining of arguments until strictly needed
2. (Maybe) re-implementing os.path.join(), as pathlib did before gh-94909: fix joining of absolute and relative Windows paths in pathlib  #95450.

Linked PRs

• GH-104996: Defer joining of pathlib.PurePath() arguments. #104999
• [3.12] GH-104996: Defer joining of pathlib.PurePath() arguments. (GH-104999) #105483
• GH-104996: Implement path joining algorithm in pathlib #105484

[barneygale]

The reason for (maybe) re-implementing os.path.join():

Any implementation of path joining must keep a running record of the current drive, root and tail, and update them by calling splitroot() on each argument. The final drive, root and tail would be useful in pathlib to implement drive, root and _tail, but os.path.join() only makes available the joined path, which is a combination of the drive, root and tail. As a consequence, we need to call splitroot() all over again in _parse_path().