gh-36369: partial cython-lint cleanup in padics/
Cleaning `pyx` and `pxi` files in `padics`, following suggestions of
`cython-lint`
### :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/36369
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
