Drop 'black' for 'ruff format' in CI

[brews]

This completes replacing black with ruff for linting and format checks in CI and dropping black entirely as a requirement from the package.

Few people will notice the change unless they're looking at PRs and results from CI.

Ruff asked for a minor code clean up, so that's in here, too.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.91%. Comparing base (0246464) to head (97cb51c).

Files	Patch %	Lines
src/dscim/utils/rff.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #271   +/-   ##
=======================================
  Coverage   68.91%   68.91%           
=======================================
  Files          15       15           
  Lines        1750     1750           
=======================================
  Hits         1206     1206           
  Misses        544      544           

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[brews]

@kemccusker I'm marking you as a reviewer as a heads-up about this change. Things might look a bit different, but it's running the same kind of thing, just faster and with one less dependency.

[brews]

Codecov is giving this the ❌ because it does a minor format change on lines that have no code coverage. Good thing to raise but I'm pretty sure it's okay.

[kemccusker]

looks fine to me, @brews, I trust you on this