Tests stalling failing on Python 3.6 in CI

[jaraco]

As early as this run, tests on Python 3.6 often fail to complete, timing out after 6h.

[jaraco]

The issue doesn't occur for me locally (on macOS nor Ubuntu Focal in container), so I'll be remote troubleshooting on GHA.

[jaraco]

Here's something weird: this run failed while this run passed at the exact same hash.

[jaraco]

The issue does appear to be related to the resolver and it affects Python 3.7 also.

[jaraco]

I now suspect this is another manifestation of pypa/pip#9143, where pytest-black or perhaps another of the test dependencies relies on importlib_metadata; python_version<"3.8" and that triggers the deep loops.

I'm going to try building a refreshed ubuntu:focal with latest tox and see if I can replicate the issue that way.

[jaraco]

As hard as I tried, I was unable to replicate the issue locally (even pinning to same pip version 20.3.1). Nevertheless, copying the workaround from pytest-checkdocs did the trick, so I'm going with that for now.