Client constructor needs API Version argument for Paho MQTT 2.x.

[jsbosch]

The current code has this:
client = mqtt.Client("Smartphone")

It needs to be changed to this:
client = mqtt.Client(2, "Smartphone")

[sneetsher]

int raised an error message. This worked for me:

client = mqtt.Client(mqtt.CallbackAPIVersion.VERSION2, "Smartphone")

Probably, it was changes in late releases.