Have all the scripts run in python 3 by default

The Python 3 refactoring effort is finishing, it should be safe
to turn all scripts to run in Python 3 by default.

https://pagure.io/freeipa/issue/4985

Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipapython/setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/python3
 # Copyright (C) 2007  Red Hat
 # see file 'COPYING' for use and warranty information
 #