Restore check for jupytersphinx in doc build

[kwankyu]

Simple fix for https://groups.google.com/g/sage-release/c/x5zO6z5c0d4/m/CougOT6mBgAJ.

Simpler and non-destructive alternative to #41142

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[dimpase]

This is not tackling the root cause of the issue at hand, which is a broken venv with missing packages, forced to be broken by a configure hack.

[github-actions]

Documentation preview for this PR (built with commit de9a697; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[kwankyu]

This PR, hopefully, fixes the direct cause of the issue at hand.

Let's wait for Marc's test.