Gracefully stop the :class:`Actor`.

        Implemented by the :meth:`.Concurrency.stop` method of the
        :attr:`concurrency` attribute.

def stop(self, exc=None, exit_code=None):
        '''Gracefully stop the :class:`Actor`.

        Implemented by the :meth:`.Concurrency.stop` method of the
        :attr:`concurrency` attribute.
        '''
        return self._concurrency.stop(self, exc, exit_code)