fix various suggestions by lgtm

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Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`d5541fe`](https://github.com/sagemath/sagetrac-mirror/commit/d5541fe58ee76b70167a9bd476f549f2a29401cc)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34342_

