Removes handler from message listeners.

        :param str message:
            Name of message to unsubscribe handler from.

        :param callable handler:
            Callable that should be removed as handler for `message`.

def unsubscribe(self, message, handler):
        '''
        Removes handler from message listeners.

        :param str message:
            Name of message to unsubscribe handler from.

        :param callable handler:
            Callable that should be removed as handler for `message`.
        '''
        with self._lock:
            self._subscribers[message].remove(WeakCallable(handler))