The simple slicing API (sq_slice, sq_ass_slice) has been deprecated
since Python 2.0 and has been removed altogether in Python 3, so remove
those functions from the PLyResult class. Instead, the non-slice
mapping functions mp_subscript and mp_ass_subscript can take slice
objects as an index. Since we just pass the index through to the
underlying list object, we already support that. Test coverage was
already in place.