gh-36413: various details in schemes
This is fixing a few details as suggested by ruff.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36413
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
