fix E714 in pyx files outside of matrix/

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

 E714 test for object identity should be 'is not'

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`fc00ba2`](https://github.com/sagemath/sagetrac-mirror/commit/fc00ba2d2705e3a188d819397599426715932d0a)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34895_

