gh-40437: cleanup and annotations in congroup_gammaH.py
    
Did some review of the file
`src/sage/modular/arithgroup/congroup_gammaH.py` to enhance the code and
add annotations.

(also a tiny change in another file)

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40437
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Travis Scrimshaw

Author: Release Manager