proton: Call python3 explicitly on Linux

Frustratingly, Ubuntu ships no python binary by default, only python3,
so we can't just call python and expect it to work. This will fail on
distros that don't ship python3, but them's the breaks.

user_settings.sample.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-
 #to enable these settings, name this file "user_settings.py"
 
 user_settings = {