Use versioned python binary.

Unversioned python binary is no more.

This reverts commit 4223ad5 "Support both python 2 and 3".

Author: prahal

Diff for: contrib/hooks/post-receive/mail_notifications.py

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