Force demo.py to use python3.

/usr/bin/python is often not defined. Choose python3 as the default
for demo.py so use doesn't get:

  env: ‘python’: No such file or directory

on startup.

Author: rouilj

demo.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import sys
 import roundup