feat(mrml-python): add a Python types stub file

[blopker]

Fix for #412

• Add a types stub file for the Python package
• Update and migrate to Py03 0.21.2

All the test files now lint correctly with Pylance in VSCode. 🎉

[jdrouet]

👍 Thanks for this, that's amazing! I reorganized your commits and started the build but this looks good to merge when the CI will be ok with it.

[blopker]

Great! Thanks for the merge. I think the doc strings leave some room for improvement, but good enough for now.