gh-38133: some care for blank lines in coding and modules
fixing pycodestyle warning about blank lines in pyx files in `coding`
and `modules`
### :memo: Checklist
- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/38133
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
