gh-39584: pep8 and mypy cleanup of arith/misc.py
    
mostly fixes for pycodestyle warnings, but also a little typing
annotation to please mypy

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #39584
Reported by: Frédéric Chapoton
Reviewer(s):

Author: Release Manager