isotesters need python3-potr and python3-slixmpp

For tails!278 (merged) (tails#17031 (closed)) we need to install python3-potr and python3-slixmpp on the isotesters. Once tails!278 (merged) 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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information