support for jupytext .py notebooks

[parmentelat]

Is your feature request related to a problem? Please describe.

I'd like to be able to use jupyter-book with (jupytext-backed) notebooks that have a .py extension, in addition to the .md extension that is currently supported

Additional context
bouncing from chrisjsewell/ipypublish#133 where I gave more context about my use case

[choldgraf]

My intuition is that we could support Python files, since that is already a common pattern for documentation and Jupytext supports them natively in a variety of formats. For other files, we could refer to the jupytext conversion docs? https://beta.jupyterbook.org/content-types/jupytext.html

[chrisjsewell]

Heya just to make some notes on this. It's definitely possible, in doing this though we should add support for all of the other extensions supported by jupytext (as of now: Rmd, py, R, r, jl, cpp, ss, clj, scm, sh, ps1, q, m, pro, js, ts, scala, rs, robot, cs, fsx, fs, sos) in the form of an optional activation, probably just via the source_suffix config option.

1. This would involve making jupytext a dependency (shouldn't be a problem)
2. Unless changes were made in jupytext, these formats would not allow for the same mapping of source line numbers for error/warning reporting as .md, i.e. for myst we ensure that errors will reference the correct line numbers of the original source file, after conversion to notebooks.
3. Users may have to add their own exclude patterns (if you add .py, will sphinx try to read conf.py?)
4. Should make sure this also works with Best way to download notebooks created from myst-notebook files? #148