fix some E251 in modular

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Component: **modular forms**

Author: **Frédéric Chapoton**

Branch/Commit: **[`18478ca`](https://github.com/sagemath/sagetrac-mirror/commit/18478cae2dc614a1e53890b6dbaee7431f107a4d)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34588_

