Optimize joining of pathlib.PurePath() arguments. #104996

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