gh-38080: fixing superfluous empty lines in libs folder

    
this is removing empty lines in the `libs` folder in `.pyx` files, for
compliance with pep8 E302 and E305

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #38080
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe