pep cleanup in parallel/

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

also some pylint and codespell fixes there

CC:  @dcoudert @kwankyu

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`f930224`](https://github.com/sagemath/sagetrac-mirror/commit/f93022471db0b5ceebb0d127a89aed8796865df4)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34815_

