gh-36142: python-style loops in `sage.graphs`
Convert the loops of the form `for x from 0 <= x < n` in `sage.graphs`
to python-style loops.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [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 accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/36142
Reported by: David Coudert
Reviewer(s): David Coudert, Frédéric Chapoton
