pathlib ABCs: defer path joining (#126409)

pathlib ABCs: defer path joining (#126409)

Defer joining of path segments in the private `PurePathBase` ABC. The new
behaviour matches how the public `PurePath` class handles path segments.

This removes a hard-to-grok difference between the ABCs and the main
classes. It also slightly reduces the size of `PurePath` objects by
eliminating a `_raw_path` slot.