Message 333546 - Python tracker

@Rémi Aye, filling out derived slots is one of the things PyType_Ready does.

This would need a discussion on python-dev before going ahead and doing it though, as the closest equivalent we have to this right now is the "negative" derivation, where overriding __eq__ without overriding __hash__ implicitly marks the derived class as unhashable (look for "type->tp_hash = PyObject_HashNotImplemented;").