title: Not receiving messages with subscribe using Paho MQTT
body: <p>I'm having a problem with subscribing to MQTT broker (using test.mosquitto.org with port 1883) with paho mqtt library. When I use mosquitto_sub client, I'm getting messages normally, but when I subscribe to the same topic in my Python script, callback never gets executed.</p>
  <pre><code>import paho.mqtt.client as mqtt
  
  client_get = mqtt.Client(client_id='my_client', protocol=mqtt.MQTTv31)
  client_get.connect('test.mosquitto.org', 1883)
  client_get.loop_start()
  
  def callback(client, userdata, message):
      print(str(message.payload.decode(&quot;utf-8&quot;)))
  
  client_get.on_message = callback
  client_get.subscribe(topic, qos=1)
  </code></pre>