flake8 repository has moved from gitlab to github

I noticed the pre-commit checks failing to clone the flake8 repository to run the relevant check. I think this is mitigated in some cases by caching, but in any case the repo appears to be on Github now, and the last few releases on PyPI point to this rather than the (no longer available) gitlab repository).

The isort hook also seems to be failing with a poetry error; I've updated that to the latest version to resolve it.