Fix CI

[njsmith]

And hopefully will fix test failures like https://github.com/python-hyper/wsproto/actions/runs/4462959392/jobs/8252677125?pr=183

[njsmith]

......okay so this is a mess. As far as I can tell:

• wsaccel has dropped support for python 2
• autobahn testsuite still depends on python 2
• autobahn testsuite requires wsaccel. so even if we install python 2, a simple pip install autobahntestsuite still fails.
• autobahn testsuite is unmaintained
• wsaccel is also unmaintained (the pypi page has a note at the top stating that this is because autobahn is unmaintained)

I think at this point we should give up on running the autobahn tests. They were really useful when we were bootstrapping to get confidence that we had things working at all, but at this point they're just breaking our CI and we've had plenty of times we've had to work around issues with them -- I don't really trust them for correctness any more than our code/tests.

[njsmith]

(we should also figure out if we want to do anything about our own wsaccel dependency, but that's a whole other issue)

[njsmith]

OK! I think this PR is finally ready!

@Kriechi I see you reviewed this just before I made the most-potentially-controversial change (removing autobahn), so I'll wait a bit to merge to give you (or anyone else) a chance to weigh in on that :-) (also b/c this currently isn't blocking anything AFAIK)