gh-38036: cython-lint for schemes
fixing a few more cython-lint suggestions in schemes
```cython-lint --ignore=E501,E231,E265 src/sage/schemes/```
### :memo: Checklist
- [x] The title is concise and informative.
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URL: https://github.com/sagemath/sage/pull/38036
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
