Use new walk_up parameter in pathlib for traversing relative paths

[TomNicholas]

We might be able to use the walk_up parameter added to pathlib.PurePath.relative_to in python 3.12 to simplify some internal logic for handling relative tree paths.

Only relevant if we can still subclass pathlib.PurePosixPath in 3.12, see https://github.com/xarray-contrib/datatree/issues/257#issuecomment-1765062809