sagemathgh-36426: cleanup of gap3 interface file

    
This refreshes the modified file, mostly for pep8 compliance

also gathering doctests annotations

also changes two calls to `super`

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: sagemath#36426
Reported by: Frédéric Chapoton
Reviewer(s): Kwankyu Lee