isotesters need python3-potr and python3-slixmpp

For tails/tails!278 (tails/tails#17031) we need to install python3-potr and python3-slixmpp on the isotesters. Once tails/tails!278 is merged we can even uninstall python-jabberbot and python-potr from the isotesters, so the automated test suite doesn't depend on python2 at all.