Gets the list of classic email groups.

def classic_email_groups(self, client_id=None):
        """Gets the list of classic email groups."""
        if client_id is None:
            response = self._get("/transactional/classicEmail/groups")
        else:
            response = self._get(
                "/transactional/classicEmail/groups?clientID=%s" % client_id)
        return json_to_py(response)