gh-37660: Fix e302 libs pyx
This is adding missing empty lines (pycodestyle E302) in pyx files in
the `modules` and `libs` folders.
Also some little tweak in code in the singular libs files.
### :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/37660
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
