dropin replace sleeksmpp with slixmpp, alter test

SleekXMPP has been discontinued for some time.

slixmpp is the actively developed fork, there are quite a few changes but it appears for our purposes the client will function as a drop in replacement.

This needs to be actively tested against an openfire server to be confident the accrued changes to slixmpp dont cause issues and require more drastic changes.

this MR also includes a small change to a test where the middleware class behaviour changed in django 4.0