Stops listening for new connections.

        Requests currently in progress may still continue after the
        server is stopped.

def stop(self) -> None:
        """Stops listening for new connections.

        Requests currently in progress may still continue after the
        server is stopped.
        """
        if self._stopped:
            return
        self._stopped = True
        for fd, sock in self._sockets.items():
            assert sock.fileno() == fd
            # Unregister socket from IOLoop
            self._handlers.pop(fd)()
            sock.close()