testing: Switch to Python 3

Both swidGenerator and strongTNC were migrated to Python 3.
This allows to migrate all testing scripts to Python 3, too.

testing/tests/route-based/net2net-xfrmi-ike/hosts/sun/etc/updown.py

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