py: Cast mp_obj_t to concrete types explicitly.
mp_obj_t internal representation doesn't have to be a pointer to object, it can be anything. There's also a support for back-conversion in the form of MP_OBJ_UNCAST. This is kind of optimization/status quo preserver to minimize patching the existing code and avoid doing potentially expensive MP_OBJ_CAST over and over. But then one may imagine implementations where MP_OBJ_UNCAST is very expensive. But such implementations are unlikely interesting in practice.