I tried to implement Nick's idea with the separate scope for __class__. It seems to work, I'm attaching a patch. The patch basically causes the following class statement:

class C(A, B, metaclass=meta):
    def f(self):
        return __class__

To be compiled approximately like this:

def _outer_C(*__args__, **__kw__):
    class _inner_C(*__args__, **__kw__):
        def f(self):
            return __class__
    __class__ = _inner_C
    return _inner_C
C = _outer_C(A, B, metaclass=meta)

It also includes some tests.

(The patch also changes the magic number in Lib/importlib/_bootstrap.py. This caused Python/importlib.h to be regenerated, but I didn't included those changes in the patch, because its a lot, and not very human-readable. Please tell me if I need to include them.)