fix E714 in pyx files inside matrix/

<div id="comment:0"></div>

about using "is not"

following #34895

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`c621882`](https://github.com/sagemath/sagetrac-mirror/commit/c6218823d5993aa3b463bdb48ff367ef3337c322)**

Reviewer: **Martin Rubey**

_Issue created by migration from https://trac.sagemath.org/ticket/34921_

