[3.13] gh-148914: Fix memoization of in-band PickleBuffer in the Python implementation (GH-149052) (GH-149275)

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.
(cherry picked from commit b89735625dff07005c31bdc86cbe7113ef1b59d0)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>