Make CI linting more useful

[ml-evs]

At some point when we switched over to GH actions, we started ignoring flake8 warnings, so they've been mounting up...

I would be in favor of leaving line length warnings muted as they will be handled by black, but there's quite a few others warning types that might need #noqa'ing in the code.

I also note that our "dev_requirements" currently contains pylint and not flake8, so we should either switch this round, move to pylint only, or start using both!

[CasperWA]

What's the advantage of using both? (Other than having two services yelling at you at once..)

[ml-evs]

AFAIK pylint has more checks for bad code smell, e.g. max function complexity and can be stricter with allowed variable names, but I haven't used it much.

I noticed that we have quite a few modules with pylint directives in them already, so presumably at some point the idea was to use pylint!