Update rpitx.py fix string to bytes

rpitx.py

@@ -294,9 +294,10 @@ if __name__ == "__main__":
 	try:
 		ser = serial.Serial("/dev/ttyAMA0", 9600)
 		print(ser.portstr)
+		uhf_string = "AT+DMOSETGROUP=0," + rx +"," + tx + ",0,3,0,0\r\n"
 		for i in range(6):
 #			ser.write(b"AT+DMOSETGROUP=0,435.0000,434.9000,0,3,0,0\r\n")
-			ser.write(b"AT+DMOSETGROUP=0," + rx +"," + tx + ",0,3,0,0\r\n")
+			ser.write(uhf_string.encode())
 			sleep(0.1)
 	except:
 		print("Error in serial write")