Add flake8 lint job to CI workflows

This PR adds a flake8 job to the github actions CI workflows, and configures flake8 in setup.cfg to only report on F codes (for now). This should highlight unused imports/variables and that sort of thing, but nothing style-related. I recommend that (if this is merged) the list of flake8 codes should be extended over time to ensure quality of the code in the repo.