GeezerVison

This looks good, other than the issue I mention below.

We will want to make both the module-level code (like this) and the operations on the weakref object thread-safe. I think it makes sense to split them up as separate PRs as you are doing.

Regarding Argument Clinic: maybe the @critical_section directive should take an optional argument specifying which parameter name to lock. For now, I think writing the critical sections manually is fine.

The tricky bit will be making _PyWeakref_GET_REF thread-safe because the lock is in the pointed-to object (ref->wr_object), but that might change or even get deallocated. I think that will need an operation _Py_TryXFetchRef from nogil-3.12 that isn't upstreamed yet. It may be worth doing the other bits and leaving _PyWeakref_GET_REF for last