Stop subclassing Path with RequestBundleDir and SourceDir (python 3.12 fix)

[eskultety]

RequestBundleDir and SourcesDir both subclass Path which is now breaking because Path (since python 3.12 pathlib) operations that return a new Path (like joinpath and relative_to) are returning a new RequestBundleDir or SourcesDir instead of a new Path like the cachito code expects.

Reference: https://github.com/python/cpython/blob/2f0778675ad0eaf346924ef6a2f60529b92ffcfa/Lib/pathlib/_abc.py#L233
Found-by: @taylormadore