Put the client in the to-be-disconnected list.

        This allows the client data structures to be present while the
        disconnect handler is invoked, but still recognize the fact that the
        client is soon going away.

def pre_disconnect(self, sid, namespace):
        """Put the client in the to-be-disconnected list.

        This allows the client data structures to be present while the
        disconnect handler is invoked, but still recognize the fact that the
        client is soon going away.
        """
        if namespace not in self.pending_disconnect:
            self.pending_disconnect[namespace] = []
        self.pending_disconnect[namespace].append(sid)