test failing due to incompatible versions for flake8-isort and isort being installed

[sanderr]

For some reason pip isn't satisfied with the latest flake8 and flake8-isort (it does install the latest isort). It tries a couple of versions until it finds one that fits the projects' requirements. In practice the installed version of flake8-isort isn't compatible with the latest isort though, even though the constraints match. At the moment I see no reason why the latest flake8 and flake8-isort don't satisfy pip. The focus of this ticket should initially be to figure out what causes this version backtracking, then to apply either a fix or a workaround.

Side note: if you would like to use pipdeptree to investigate this, install pip<21.3 (tox-dev/pipdeptree#153).

[sanderr]

Waiting for response on sarugaku/resolvelib#91.