encode the output string

robot.py

@@ -35,9 +35,9 @@ class Robot:
     def move(self, coords=None):
         if coords is not None:
             self.target = coords
-        str = str.lower()
         hexval = [hex(dec).lstrip("0x").rstrip("L") for dec in self.target]
         str = '{'+','.join(hexval)+'}'
+        str = str.lower().encode()
         try:
             self.serial_port.write(str)
         except serial.serialutil.SerialTimeoutException: