gh-36374: refresh the cython file real_roots (pep8, range, etc)
    
just refreshing a little bit the code in the modified file

- using range at some place (partial)
- fix all cython-lint warnings

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36374
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert, Frédéric Chapoton

Author: Release Manager