gh-38137: some care for blank lines in pyx in rings
some care for blank lines in pyx files in `rings` (only partial here)
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/38137
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
