Tests fail with `flake8>=5.0`

[StokesMIDE]

At some point in Q1 2022, an update of flake8 made some tests fail. It is likely indirect, caused by something in pytest-flake8. A workaround was created by pinning flake8<5.0, but it's a hack.

If future updates to flake8 and/or pytest-flake8 don't fix the issue, we'll have to make a change, possibly to our CI/CD pipeline.

Will add more information if/when time allows.