gh-37965: some polishing for pbori (ruff and pep8 mostly)
just a basic polish of the ` pbori` folder, fixing pep8 warnings and
some ` ruff` suggestions
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/37965
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
