Stop and close the DTLS transport.

async def stop(self):
        """
        Stop and close the DTLS transport.
        """
        if self._task is not None:
            self._task.cancel()
            self._task = None

        if self._state in [State.CONNECTING, State.CONNECTED]:
            lib.SSL_shutdown(self.ssl)
            try:
                await self._write_ssl()
            except ConnectionError:
                pass
            self.__log_debug('- DTLS shutdown complete')