Implement require_hash in the get method of Jupytext's content manager

[mwouts]

Will close #1165

TODO:

• Find out how to properly combine two hashes
• Don't add a require_hash argument if the original contents manager does not have one
• Add a test against jupyter_server==2.10.x

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

pip install git+https://github.com/mwouts/jupytext.git@implement_require_hash

(this requires nodejs, see more at Developing Jupytext)