PurePathBase shouldn't cache drive/root/tail/string

[barneygale]

PurePathBase has a few slots and methods that facilitate fast path object generation:

• _drv, _root, _tail_cached: roughly, the result of os.path.splitroot() on the path
• _str: the normalized path string
• _make_child_relpath(): used when walking directories
• _from_parsed_parts(): used in parent, parents, with_name(), relative_to()

These features are useful in PurePath, where speed is important, but much less so in PurePathBase, where the clarity of the interfaces and interactions should win out.

It should be possible to move them to PurePath, leaving PurePathBase with only _raw_paths and _resolving slots.

[barneygale]

This might also make it possible to implement PathBase.resolve() without spinning up loads of path objects.

[barneygale]

Upstream issue: python/cpython#113528