gh-39113: cython-lint cleanup for one pxi file in matrix
    
just fixing all warnings of

`cython-lint --ignore=E501,E741
src/sage/matrix/matrix_modn_dense_template.pxi`

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #39113
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager