gh-38644: pathlib in simplicial sets examples
use `pathlib.Path` instead of `os` in the modified file
plus a few pep8 changes there
### :memo: Checklist
- [x] The title is concise and informative.
- [x] 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.
URL: https://github.com/sagemath/sage/pull/38644
Reported by: Frédéric Chapoton
Reviewer(s): Kwankyu Lee
