Retransmit an RTP packet which was reported as lost.

async def _retransmit(self, sequence_number):
        """
        Retransmit an RTP packet which was reported as lost.
        """
        packet = self.__rtp_history.get(sequence_number % RTP_HISTORY_SIZE)
        if packet and packet.sequence_number == sequence_number:
            if self.__rtx_payload_type is not None:
                packet = wrap_rtx(packet,
                                  payload_type=self.__rtx_payload_type,
                                  sequence_number=self.__rtx_sequence_number,
                                  ssrc=self._rtx_ssrc)
                self.__rtx_sequence_number = uint16_add(self.__rtx_sequence_number, 1)

            self.__log_debug('> %s', packet)
            packet_bytes = packet.serialize(self.__rtp_header_extensions_map)
            await self.transport._send_rtp(packet_bytes)