__module__ attribute is set differently depending on whether a metaclass is explicitly called or invoked in a class statement:

>>> A = ABCMeta('A', (), {})
>>> A.__module__
'abc'
>>> class B(metaclass=ABCMeta): ...
... 
>>> B.__module__
'__main__'

Documentation on data model says that "__module__ is the module name in which the class was defined", so that the second behaviour seems right, while the first behaviour seems wrong to me.