gh-38443: fix E302 in pxi files
just for the sake of house cleaning
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
- [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/38443
Reported by: Frédéric Chapoton
Reviewer(s): Martin Rubey
