Update scaffolding

[madig]

Should fix CI.

• Test with Python 3.9 to 3.13, older Pythons may not be available anymore on CI runners.
• Replace the old setup.py setup with a pypackage.toml. I saw various deprecation warnings when building packages, so I think it's better to do this now rather than when something breaks down the road. I opted to use uv to do project managment and packaging because it's the best tool for the job :)

[coveralls]

coverage: 87.668%. remained the same
when pulling fbdc4bd on daltonmaag:fix-ci
into 4c0bc02 on unified-font-object:master.

[coveralls]

coverage: 87.668%. remained the same
when pulling fbdc4bd on daltonmaag:fix-ci
into 4c0bc02 on unified-font-object:master.

[coveralls]

coverage: 87.499% (-0.2%) from 87.668%
when pulling a935f28 on daltonmaag:fix-ci
into 4c0bc02 on unified-font-object:master.

[madig]

I don't know why coveralls keeps failing. The comment TheKevJames/coveralls-python#387 (comment) suggests that maybe Coveralls needs to freshen up its data? @benkiel Could you please have a look? Or try rerunning all jobs?