The function PyCFunction_GetFunction(func) check func type should be
exactly PyCFunction_Type. PyCFunction_GET_FUNCTION(func) does not check
func type. But assumes func layout starts with PyCFunctionObject. i.e.
for allowing func to be PyCFunction_Type subclass.

This way an extension which subclasses PyCFunctionObject will get
into trouble using PyCFunction_GET_FUNCTION() on Pyston