Retire jupytext, in favour of GitHub 'rich notebook diff'

[pp-mo]

From my ultra-simple experiment here , I think this really shows promise.
I think using this could really simplify development -- I find I need to fix/correct the jupytext approach quite a lot, especially in merges.

Though it is an experimental feature -- you must turn in on in your personal HG profile.
Details here : https://github.blog/changelog/2023-03-01-feature-preview-rich-jupyter-notebook-diffs/