modern super in matrix_space

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and other pep8 details

Component: **linear algebra**

Author: **Frédéric Chapoton**

Branch/Commit: **[`78ada7d`](https://github.com/sagemath/sagetrac-mirror/commit/78ada7d31093789608bfbb8687844cb664e40bed)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34469_

