gh-116738: Make zlib module thread-safe (gh-142432)

gh-116738: Make zlib module thread-safe (gh-142432)

Makes the zlib module thread-safe free-threading build. Even though operations
are protected by locks, attributes exposed via PyMemberDef (eof, needs_input,
unused_data, unconsumed_tail) should still be stored atomically within locked
sections, since they can be read without acquiring the lock.