Lint cleanliness

[sajith]

When ran against fablib, flake8 reports a list of errors and warnings. It would be a good idea to fix them.

This will be a two-part process:

• [ ] Add flake8 linting as a CI step.
• [ ] Address flake8 errors and warnings, while using the CI step to validate that we've indeed addressed them correctly.

Right now, running flake8 in a single workflow where we check out source code, install dependencies, and run pytest is a little tricky: fim has pycodestyle==2.6.0 as a dependency, but there's no flake8 release that can use that particular version of pycodestyle. We can add a separate workflow for linting, but, in order to avoid duplication, I'd like to keep the number of workflows minimal.

[sajith]

https://github.com/astral-sh/ruff is newer, faster, and claims parity with flake8+isort+black. Adopting ruff (and dropping isort and black checks from CI) would be a good idea, since ruff should give us more useful feedback, and hopefully be less annoying than dealing with just isort and black checks.