Return a list of slots and methods, including those from subclasses.

def _superdir(self):
        """
        Return a list of slots and methods, including those from subclasses.
        """
        attrs = set()
        cls = self.__class__
        if hasattr(cls, '__all_slots__'):
            attrs.update(cls.__all_slots__)
        for bcls in cls.__mro__:
            if hasattr(bcls, '__dict__'):
                attrs.update(bcls.__dict__)
        return attrs