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/strongswan.conf

@@ -7,6 +7,6 @@ swanctl {
 charon-systemd {
   load = random nonce aes sha1 sha2 pem pkcs1 curve25519 gmp x509 curl revocation hmac vici kernel-netlink socket-default
   start-scripts {
-    updown = /usr/bin/python /etc/updown.py
+    updown = /usr/bin/python3 /etc/updown.py
   }
 }

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