demo for mwouts/jupytext#900

to reproduce the bug:

• setup:
  • create a virtualenv
  • install requirements.txt
  • install pre-commit and run pre-commit install (or, you can just run jupytext on the command line manually)
• reproduce:
  • run jupyter-lab
  • open jupy_test.ipynb, make a change, and save.
    • this should auto-update jupy_test.py.
    • git diff should show that jupyt_test.ipynb now has a metadata.jupytext section.
  • (via pre-commit) git commit -a -m 'test' triggers jupytext. this removes the newly added metadata.jupytext section, causing pre-commit to fail. running git commit -a again succeeds.
  • (via jupytext manually) run jupytext --sync jupy_test.ipynb and note that the metadata.jupytext section was removed from the notebook