fix and activate W291 in py files

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

this is about "no trailing whitespace"

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`68cef16`](https://github.com/sagemath/sagetrac-mirror/commit/68cef160ba45dc79054e1a3723f05c202a5f8da4)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34785_

