object.h:  Added PyType_CheckExact macro.

typeobject.c, type_new():

+ Use the new macro.
+ Assert that the arguments have the right types rather than do incomplete
  runtime checks "sometimes".
+ If this isn't the 1-argument flavor() of type, and there aren't 3 args
  total, produce a "types() takes 1 or 3 args" msg before
  PyArg_ParseTupleAndKeywords produces a "takes exactly 3" msg.