Irreversibly stop the sender.

async def stop(self):
        """
        Irreversibly stop the sender.
        """
        if self.__started:
            self.__transport._unregister_rtp_sender(self)
            self.__rtp_task.cancel()
            self.__rtcp_task.cancel()
            await asyncio.gather(
                self.__rtp_exited.wait(),
                self.__rtcp_exited.wait())