Tooling: drop flake8, configure ruff

[dbarnett]

Why: Ruff is more powerful, flexible, and faster than flake8, with better defaults and integrated formatter.

FYI I also added corresponding editorconfig settings to help ensure that editors and formatters aren't fighting each other over the style defaults, but it's technically independent (and actually ignored by ruff currently ☹️).

[dbarnett]

BTW my motivation is more to get off of flake8 than to rush to get onto ruff, but I like what I've seen of ruff so far and tox config makes it fairly painless to switch dependencies.

The set of checks I enabled seemed like a pretty uncontroversial set of basic checks, but we can totally customize later if any of them are giving people grief.

Also note I didn't batch reformat files through ruff and I'm not planning to try to enforce strict formatting policies or anything, but I did peek through the diff of formatting changes ruff would want to perform if we asked it to reformat everything and adjusted some settings to try to keep that suggested formatting pretty close to the existing versions where possible.

[dbarnett]

@mdengler FYI I went ahead and submitted the .editorconfig changes separately while I was waiting for review here. Mostly just looking for a gut check that switching to ruff looks reasonable.

[dbarnett]

Gonna go ahead and merge this one too. Can easily revert it later if needed.