pep8 cleanup in ore_polynomial_element.pyx

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

also fixing one "unreachable code" compilation warning

CC:  @tscrim @dcoudert

Component: **algebra**

Author: **Frédéric Chapoton**

Branch/Commit: **[`bbe553a`](https://github.com/sagemath/sagetrac-mirror/commit/bbe553a1397fd2683c5f970475091bd6e9aaf658)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34843_

