gh-40789: full pep8 cleanup of order_polymod
    
plus a few minor code enhancements

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40789
Reported by: Frédéric Chapoton
Reviewer(s): Vincent Macri

Author: Release Manager