gh-36270: full pep8 for modular/hecke
small cleanup for `modular/hecke` folder
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36270
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
