Obtain a :class:`.Relationship` subclass for a given
        relationship type name.

def relationship_type(self, name):
        """ Obtain a :class:`.Relationship` subclass for a given
        relationship type name.
        """
        try:
            cls = self._relationship_types[name]
        except KeyError:
            cls = self._relationship_types[name] = type(str(name), (Relationship,), {})
        return cls