Make jupytext an optional dependency

[martinRenou]

Problem

I'd like to suggest that we make jupytext an optional dependency.

The problem I see is that jupytext is incompatible with JupyterLite today, and pulling it in the environment pulls the labextension for it, that JupyterLite will pick up automatically, even though it's only needed at build time here (for processing markdown files).

@agriyakhetarpal what do you think?

[agriyakhetarpal]

Hi @martinRenou, this sounds good to me. Some context is over at #258 – jupytext was meant to be optional (and I wanted it to be when I added support in #191!), but no one responded to it in the PR review, so it was left out. Sorry that I missed your comment over there!

As an alternative, one option that I realised later on to counter this issue is for jupyterlite-sphinx to ship a jupyter-lite.json file with jupytext disabled via the disabledExtensions key, and we coalesce the contents of any project-specific jupyter-lite.json files with ours. I don't know if this is possible with JupyterLite, so perhaps making it an optional dependency is best.

[agriyakhetarpal]

P.S. I am also following mwouts/jupytext#1225, though it is not necessary for this issue as there is still value in making jupytext optional. How do you feel about [markdown] as the optional dependency set name?

[martinRenou]

How do you feel about [markdown] as the optional dependency set name?

Sounds good to me 👍🏽

On the conda-forge side, we could make it an extra package in the recipe e.g. jupyterlite-sphinx-markdown.

[agriyakhetarpal]

That works!