bpo-35615: Fix crashes when copying a Weak{Key,Value}Dictionary. (GH-11384)

Protect dict iterations by wrapping them with _IterationGuard in the
following methods:

- WeakValueDictionary.copy()
- WeakValueDictionary.__deepcopy__()
- WeakKeyDictionary.copy()
- WeakKeyDictionary.__deepcopy__()