In its latest release, pytest-asyncio began modifying the default asyncio event_loop_policy which broke tests here as we need to explicitly set our event loop policy

There is a discussion of how to proceed on the pytest-asyncio side (xref pytest-dev/pytest-asyncio#168) but for now we should just set an upper limit on our allowed version of pytest-asycnio

Tests on Windows (mostly) pass 🎉 I've been test_tls_specific fail before on master (xref https://github.com/dask/distributed/runs/1332842736#step:8:116) and opened up an issue for tracking the failure (#4215)

Thanks.

We'll want to backport this to 2.30.x as well once it's merged.

Merging as the two test failures are known flaky tests