sagemathgh-36683: refresh the file plural.pyx

    
The main goal is to get rid of `ParentWithGens` there, replaced by
`Ring`.

I also took the opportunity to clean cython-lint warnings and other code
and doc details.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: sagemath#36683
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe