make sure you are running systemd, if not, apt-get install it:

systemd --version

if nothing:

sudo apt-get install -y systemd

test your python script, once you are confident it works properly, note its location

create your service with nano:

sudo nano /etc/systemd/system/servicename.service 

edit in the contents:

[Unit]
Description=you script's description
After=multi-user.target
[Service]
Type=simple
Restart=always
ExecStart=/usr/bin/python3 /full-path/location-to/your-script.py
[Install]
WantedBy=multi-user.target

note: the execstart line has a command to pass a space and a location, it is easy to wipe out this entire line when copy/pasting and forgetting to add the python3 part back

make it work, make it so your service starts after reboot, and finally start your service:

sudo systemctl daemon-reload
sudo systemctl enable test.service
sudo systemctl start test.service