Python 2.7 tests are failing

[hkage]

This PR addresses various issues besides the failing Python 2.7. CI tests:

• The Github actions now use different container images for testing.
• Changelog and contributing docs are now written in Markdown (for easier syntax).
• The obsolete Github links (after renaming the project) within the contributing docs are fixed.
• The minimal Dockerfile now uses the Python 3.12 slim image.
• Added tests for Python 3.12
• Use Codecov for the coverage report

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️