Infrastructure updates

[astrofrog]

This migrates the configuration from setup.cfg to pyproject.toml, adds ruff, sp-repo-review, and implements many of the suggestions of the latter.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (908be0d) 76.50% compared to head (1860773) 75.00%.

Files	Patch %	Lines
extension_helpers/_openmp_helpers.py	88.88%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #68 +/- ## ========================================== - Coverage 76.50% 75.00% -1.51% ========================================== Files 4 4 Lines 315 316 +1 ========================================== - Hits 241 237 -4 - Misses 74 79 +5 ```

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[astrofrog]

It's such a small test suite that I don't think there is any harm in doing coverage for all?

[neutrinoceros]

@astrofrog I may not be able to do a proper review for a couple days. I'm happy to come back then, but if you're in a hurry you have my blessing tu I merge.

[neutrinoceros]

(I accidentally hit the "approve" button, but I'd still like my comments addressed. Obviously I'm in favour of this PR)

[astrofrog]

@pllim @neutrinoceros - I think I have implemented your comments, this is ready for a final review!

[astrofrog]

(rebased following merging of other PRs)

[astrofrog]

@pllim - does this seem ok now?

[pllim]

Errr the conda jobs are failing... Was it my fault?

ERROR: pyproject.toml file found.

[astrofrog]

I'll look into it later this week!

[pllim]

@astrofrog , I took the liberty to push a couple of commits here. CI is green now but I'll let you have a look. If you are okay with my changes, feel free to merge. Thanks!

[astrofrog]

Thanks!