GH-103525: Improve exception message from `pathlib.PurePath()` (GH-103526)

Check that arguments are strings before calling `os.path.join()`.

Also improve performance of `PurePath(PurePath(...))` while we're in the
area: we now use the *unnormalized* string path of such arguments.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>