Add casts (one needed, one for consistency).

FWIW, I feel that these casts are misguided. Having function pointer casts means that type errors in the signature get suppressed. E.g. people using such casts for tp_hash will be unable to detect the change in the tp_hash return type that we have implemented for 3.2. It would be better, IMO, if these casts get avoided everywhere, even if that means that the functions get a line longer. Regards, Martin