Use custom format file as parent in nested TOC

[lesteve]

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

We are using .py files as custom notebook format as described in https://jupyterbook.org/file-types/jupytext.html#file-types-custom.

I bumped recently into the limitation that you can not to use custom format notebook as parent in the TOC, i.e. if you have a TOC like this

- file: index

- part: The Predictive Modeling Pipeline
  chapters:
  - file: python_scripts/02_numerical_pipeline
    sections:
      - file: python_scripts/02_numerical_pipeline_ex_01

you will get the error Only markdown, ipynb, and rst files are supported in the TOC.

Note this only happens if there is nested sections, so this TOC works fine:

- file: index

- part: The Predictive Modeling Pipeline
  chapters:
  - file: python_scripts/02_numerical_pipeline

Describe the solution you'd like

This is not a show-stopper but being able to use custom notebook format as parent in the TOC would be nice. I would be curious to know whether there is some inherent difficulty in tackling this edge case or whether it is more that nobody has asked about it (until now 😉).

Additional context

To reproduce the problem (in case it helps):

git clone https://github.com/lesteve/scikit-learn-mooc --branch custom-file-toc-limitation
cd scikit-learn-mooc/jupyter-book
jb build .

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[choldgraf]

oops - yep that should be fixable, I think we want to add a check for the "custom notebook formats" here:

https://github.com/executablebooks/jupyter-book/blob/master/jupyter_book/toc.py#L144

[chrisjsewell]

In #1293 there will no longer be this restriction on source format