gh-159564: Make weakref dereference lock-free.

[nascheme]

In the free-threaded build, change _PyWeakref_GET_REF() so it no longer takes the striped weakref list lock. The referent's memory stays readable if it is freed concurrently because object heaps delay page reuse with QSBR, and wr_object only ever transitions from the referent to Py_None, so _Py_TryIncrefCompare() is sufficient.

Use release store for clear_weakref_lock_held(), instead of sequentially consistent stores. The weakref list lock orders the list updates, and the lock-free read only requires that the store of Py_None happens before the referent's memory can be reused.

The benchmark results below are based on a C extension implementing a weak-map using either PyUnstable_TryIncRef() (borrowed refereces) vs using weak references. Thread scaling of "get, N threads, private maps" (ns per op, lower is better):

Threads	tryincref	weakref	weakref opt
1	11.1	20.8	11.7
4	4.9	21.1	5.4
8	3.0	23.0	3.2

• Issue: Avoid the weakref list lock when dereferencing weakrefs #159564