Jupytext notebook support

[choldgraf]

Currently, we only support text-based notebooks via the {code-cell} directive and {eval} role. Jupyter Book v1 supports any file format that Jupytext supports, which is a wide variety of text-based notebook formats.

I don't see any mention of Jupytext support in the docs, so assume that it isn't supported right now.

Suggestion

We should either:

• Support Jupytext formats as a general principle for notebook execution and reading.
• Define the few jupytext formats that are most common and support them
• Explicitly agree to drop jupytext support in v2.