gh-36635: clean the file hexad.py
various little pep8 details, some fixes by ruff, unicode art, etc
### :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/36635
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert, Frédéric Chapoton
