Guido, Yury, it looks like I solved the puzzle. All the remaining problems are because of forward references. In particular, _ForwardRef keeps a reference to the frame where it was defined:

        typing_globals = globals()
        frame = sys._getframe(1)
        while frame is not None and frame.f_globals is typing_globals:
            frame = frame.f_back
        assert frame is not None
        self.__forward_frame__ = frame

This is old code from 2015 introduced to support __isinstance__ for forward refs. The latter is no more supported anyway, so that I believe this code should be removed.

I will make a PR upstream soon.