gh-148914: Fix memoization of in-band PickleBuffer in the Python implementation ...

gh-148914: Fix memoization of in-band PickleBuffer in the Python implementation (GH-149052)

Previously, identical PickleBuffers did not preserve identity.
Also, empty writable PickleBuffer memoized an empty bytearray object
in place of b'' which is a singleton in CPython, so the following
references to b'' were unpickled as an empty bytearray object.