to start up automatically, add executing the to your rc.local (or whatever script runs at startup)
replace paths with your own locations.

for your /etc/rc.local file:
/usr/bin/env python /home/hlmtre/src/brightness/

for your i3 config: (replace path, obv)

 bindsym XF86MonBrightnessDown exec python /home/hlmtre/src/brightness/ down
 bindsym XF86MonBrightnessUp exec python /home/hlmtre/src/brightness/ up

the shell script must be run as root, as well as the the client can run as a regular unprivileged user.