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There is one test (ClassPropertiesAndMethods.test_mutable_bases_with_failing_mro in test_descr) that crashes with the code from issue551412 because _PyType_Lookup() is recursive called from PyType_Ready(). Is this the reason? My patch prevents recursive calls.
Here is minimal example (for Python 3):
class M(type):
 def mro(self):
 hasattr(self, 'foo')
 return type.mro(self)
class C(metaclass=M):
 pass
When class C is created, C.mro() is called while C still is not ready. Resolving an attribute calls _PyType_Lookup() which calls PyType_Ready() which calls mro() etc.