Thanks for the report. Upstream maintainer here. This is caused by aioxmpp poking dangerously close to some asyncio internals and Python 3.7 handling things slightly different apparently. I found this issue already while testing 3.7 in travis CI, but I did not realize that it also applies to Debian buster already.
I will try to make a 0.10.2 this year. (For reference, here’s the patch I created to fix it: https://github.com/horazont/aioxmpp/commit/ b1037556805321c0348a48af1517760871d596d3 )
