Use python2 shebang

Some distributions have switched to python3 as default and our code
breaks because it's python2 specific. Follow the PEP 394
recommendation.

tests/data/sample.activity/setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 from sugar3.activity import bundlebuilder
 

tests/test_mime.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright (C) 2006, Red Hat, Inc.
 # Copyright (C) 2007, One Laptop Per Child