Reject incompatible baseclass definitions

[JukkaL]

When using multiple inheritance, the type checker does not complain if base classes have incompatible definitions. For example, this is not rejected:

import typing
class A:
    def f(self, x: int) -> None: pass
class B:
    def f(self, x: str) -> None: pass
class C(A, B): pass # No error, but should complain about f

Verify that all inherited definitions are compatible.

[JukkaL]

Related test case: testMethodNameCollisionInMultipleInheritanceWithIncompatibleSigs-skip in check-multiple-inheritance.test.

[JukkaL]

Mypy now complains about this -- seems to work.