gh-36392: a few details in combinat, as suggested by ruff
this is fixing a few things in combinat, as suggested by the very fast
linter `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/36392
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
