Maintenance updates + various improvements

[behackl]

I actually just wanted to cleanup the workflow files, but then started to add further improvements as well, including a bump to the version of the dependencies. I'd stronly recommend using a proper dependency management tool; my proposal in this PR is to use uv.

Summary of changes:

• Introduce pyproject.toml as a central host for all project metadata
• Packaging: switch build system from setuptools to hatch/hatchling
• Bump minimum Python version to 3.9
• Switch linting from flake8 to ruff check (with similar, perhaps slightly stricter settings)
• Use ruff format in dedicated linting pipeline (although it would probably make sense to include a corresponding pre-commit hook, otherwise the linting pipeline will just fail constantly...)