Consider:

class M(type):
    def foo(self): pass
class C(metaclass=M):
    pass
print(C.__class__)
C.foo()
class D(int, C):
    pass
print(D.__class__)
D.foo()

Running this with Python 3 shows that D's metaclass is M, but when checking this with mypy, we get the error

u.py:10: error: "D" has no attribute "foo"

on the last line, showing that mypy doesn't believe so.

This seems unique to int (or perhaps builtin types?) -- when create a separate class A (without a metaclass) and use that instead of int, there is no error.

FWIW this prevents me from fixing #2305 (iterating on enums) -- I can get it to work for Enum but not for IntEnum, because it multiply inherits from int and from Enum.