build(clint): change shebang to python3

There are distributions that doesn't symlink python to python3.
This will make clint work for these without any additional steps from
the user.

src/clint.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # vim: set fileencoding=utf-8
 #
 # Copyright (c) 2009 Google Inc. All rights reserved.