Added free threading support to PyUpb_WeakMap

Previously we had a lock in the global object cache (an instance of `PyUpb_WeakMap`), but this CL refactors `PyUpb_WeakMap` to be internally synchronized.

We also add code to handle a tricky edge case that can occur when a dealloc operation races with lookup.  This implementation mirrors the code previously implemented in C++ (`PyWeakValueMap`).

PiperOrigin-RevId: 921534474