Add ruff for lint & formatting checks; apply some suggested fixes

[eloquence]

Adds a GitHub action that runs format & lint checks.

Draws from freedomofpress/securedrop#6961 for the ruff config.

I've implemented several of the linter suggestions while ignoring others.

This PR also applies ruff's code formatting (which in turn is a drop-in replacement for black). ruff appears to be a bit less aggressive than black in enforcing line length, so we have to disable the line length check itself.

NB: Our undocumented use of test-requirements.txt is another potential candidate for Poetry; I've saved this for a follow-up change.

Fixes #463

Testing

• Visual review
• Review output of new "checks" GitHub actions
• Install test-requirements.txt in a venv and verify that Makefile targets work as expected

[legoktm]

Thanks for venturing into the brave new world of ruff format :) Left some notes but overall I'm excited