Add mandatory endianness argument to the .to_bytes() method

Thanks to @st3veV

Author: Daniel Campora

lib/mqtt/mqtt.py

@@ -136,7 +136,7 @@ def subscribe(self, topic, qos=0):
         #print(hex(len(pkt)), hexlify(pkt, ":"))
         self.sock.write(pkt)
         self._send_str(topic)
-        self.sock.write(qos.to_bytes(1))
+        self.sock.write(qos.to_bytes(1, 'little'))
         while 1:
             op = self.wait_msg()
             if op == 0x90: