* Installing dependencies on a stock Raspbian install:

    sudo apt-get update
    sudo apt-get upgrade
    sudo apt-get install python3-pip python3-dev python3-gpiozero 
    sudo pip3 install paho-mqtt

* Create the python script as /home/pi/mqtt_fan.py
* Create a systemd service definition file as /lib/systemd/system/mqtt_fan.service