Inconsistent treatment of H1 headers in LaTeX build

Describe the bug Some H1 headers in .ipynb Markdown cells are converted to LaTeX \chapter{...} commands and some are converted to \section{...} commands. I can’t seem to tell or control why/how this happens.

To Reproduce Steps to reproduce the behavior:

1. Choose a jupyter-book project. I’m using this one.
2. Ensure that project has many different H1-type headers in it (using Markdown’s # in .ipynb cells), spread over several files.
3. Run jupyter-book build . --builder pdflatex.
4. View the generated PDF in ./_build/latex/. (In the above repo, it’s right here).
5. The first # header seems to have become a chapter and all other # headers have become sections within it.

Expected behavior Each H1 header should be treated equally, each creating its own chapter.

Environment (please complete the following information):

• Python Version 3.7.3
• Output of jupyter-book --version

Jupyter Book: 0.7.0    # but I've tried with 0.7.2dev0 also and got the same problem
MyST-NB: 0.8.1
Sphinx Book Theme: 0.0.23
MyST-Parser: 0.8.1
Jupyter-Cache: 0.2.1

[nathancarter]

This is great! Thanks for the persistence at it.

[AakashGfude]

thanks @RojerGS checking your issue810 repo against PR #1167 produces the following output python.pdf

Amazing work @mmcky and everybody. Can I get rid of my repository now?

I think you can. This PR is has handled the extension use case as well: https://github.com/executablebooks/jupyterbook-latex/pull/45