Remove flakehell now that it's no longer being maintained
https://github.com/life4/flakehell was added in !1970 (merged), but the maintainer has now archived the project: https://github.com/wemake-services/wemake-python-styleguide/issues/1817.
Since it's no longer being maintained, we should move away from it and start using flake8 --diff
directly.
The docs at https://wemake-python-stylegui.de/en/latest/pages/usage/integrations/legacy.html will likely come in handy as well.
Edited by Douwe Maan