Three's a charm: yet another fix for SF bug 551412. Thinking again

Three's a charm: yet another fix for SF bug 551412.  Thinking again
about the test case, slot_nb_power gets called on behalf of its second
argument, but with a non-None modulus it wouldn't check this, and
believes it is called on behalf of its first argument.  Fix this
properly, and get rid of the code in _PyType_Lookup() that tries to
call _PyType_Ready().  But do leave a check for a NULL tp_mro there,
because this can still legitimately occur.

I'll fix this in 2.2.x too.