Mypy does not detect metaclass conflicts

[sobolevn]

Minimal reproducer:

class MyMeta1(type): ...
class MyMeta2(type): ...
class A(metaclass=MyMeta1): ...
class B(metaclass=MyMeta2): ...
class C(A, B): ...

In runtime it causes:

» python ex.py
Traceback (most recent call last):
  File "ex.py", line 5, in <module>
    class C(A, B): ...
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

But, mypy is fine with it:

» mypy ex.py --strict
Success: no issues found in 1 source file

Important note: I think this should only work outside of stubs, because we do some extra metaclass hackery (like ABCMeta) in stubs. Moreover, in stubs - it won't raise.

Related #13561