GeezerVison

There's a couple of remaining issues with doing a setaddr. If we have a split dictionary we're not increfing the dictionary. Also, when we insert a split key it takes a lock which can cause the object/dict locks to be released so we need to guard against that.

The code here is pretty complex so it's also an attempt to simplify it. _PyObject_StoreInstanceAttribute loses any Py_GIL_DISABLEd ifdef's, instead we just have the main path with Py_BEGIN_CRITICAL_SECTION and FT_ATOMIC_LOAD_UINT8 and so on just turn into nothing when we're in the free threaded build.

I've also just updated things to go directly to _PyDict_SetItem_LockHeld when we're doing the dictionary version and made store_instance_attr_lock_held only be used when we don't have a materialized dictionary. Going through the fast split dictionary case it dictinsert isn't substantially different but it really complicates reasoning about what's going on here and causes `store_instance_attr_lock_held to need to handle a lot more cases.

• Issue: 22 free-threading race conditions #149816