fix various linter errors

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This should make the linters on [GitHub](../wiki/GitHub) happy.

CC:  @fchapoton

Component: **refactoring**

Author: **Lorenz Panny**

Branch/Commit: **[`1b47457`](https://github.com/sagemath/sagetrac-mirror/commit/1b4745764a283f35f6b4f85360fdae3f21033ac8)**

Reviewer: **Frédéric Chapoton**

_Issue created by migration from https://trac.sagemath.org/ticket/34466_

