[mypyc] Fix crash when '__enter__'/'__exit__' inherited from Python class

[JukkaL]

Fixes mypyc/mypyc#1193.

Compiling a with statement crashed if the context manager was a native class that inherited __enter__ or __exit__ from a non-native class:

from contextlib import AbstractContextManager

class C(AbstractContextManager):
    def __exit__(self, exc_type, exc_value, traceback) -> None:
        pass

with C() as c:  # KeyError: "'C' has no attribute '__enter__'"
    pass

Now the methods are only called natively if both are defined in a native class.