Remove `jupytext` installation steps from the GitHub Actions workflows

[milanmlft]

Once #22 is resolved, we can remove the jupytext set up steps from the GHA workflows, as this will be handled internally using the jupytextR package.

• [x] #22
• [x] Remove jupytext installation from the workflows in sandpaper/.github/workflows/
• [x] Remove jupytext installation from the lesson template worfklows (sandpaper/inst/workflows/)
• [x] Remove jupytext installation from the workflows for the existing L2D lessons:
  • [x] Basic_Python
  • [x] Data_Handling
  • [x] ML_supervised
  • [x] ML_unsupervised
  • [x] Networks