gh-116738: Make zlib module thread-safe

[yoney]

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.

• Replaced PyThread_type_lock with PyMutex (similar to gh-116738: Use PyMutex for bz2 module #140555 and gh-116738: Use PyMutex for lzma module #140711)
• Removed ENTER_ZLIB/LEAVE_ZLIB macros in favor of direct PyMutex_Lock/Unlock (similar to gh-116738: Use PyMutex for bz2 module #140555 and gh-116738: Use PyMutex for lzma module #140711)
• Used FT_ATOMIC_STORE_CHAR_RELAXED for eof and needs_input flag updates inside locked sections (attributes exposed via PyMemberDef)
• Moved unused_data/unconsumed_tail from PyMemberDef to PyGetSetDef

cc: @mpage @colesbury

• Issue: Audit all built-in modules for thread safety #116738