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Thanks for the review!
I've updated my patch:
- renamed it to _PyType_CalculateMetaclass
- in __build_class__ call it even when a metaclass is declared
- added a test for this case (which fails with my previous patch)
However I noticed another problem: the declared metaclass (the object passed with the metaclass keyword in the class definition) according to PEP 3115 can be any callable object, not only a PyTypeObject. Problems:
1. In this case, PyType_IsSubtype will be called on something that is not a PyTypeObject (I don't know if that's a big problem, currently it seems to work).
2. The bigger problem: a simple construct, like:
class X(object, metaclass=func):
 pass
(where func is for example a function) won't work, because in _PyType_CalculateMetaclass it will detect, that func isn't a super- or subtype of object.__class__, and will raise an exception: "metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases".
My first idea to solve this problem is to ignore this case in __build_class__ (check for a returned NULL, and call PyErr_Clear), and use the declared metaclass. (I don't know, if this can cause other problems, I haven't thought much about it yet.)