Replace flake8 / pytest-flake8 with ruff / pytest-ruff

[ff137]

Flake8 is being surpassed by an extremely fast linter: https://github.com/astral-sh/ruff.
See the readme for speed comparison and testimonials.

This is primarily suggested because pytest-flake8 is no longer maintained, binding aca-py to flake v4 instead of the latest v6.
To shed technical debt, I propose replacing the pytest-flake8 dependency (used in PR tests) with pytest-ruff

[dbluhm]

I am generally in favor of a change. I've heard of ruff before but this is the first time I've looked closer; I notice that the version is 0.0.282. That's the only thing that gives me pause; however, it seems to be widely used and well supported. I think I'm in favor 🙂