cleanup for QQ-curves

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mostly pep8 details

also adding one doctest

Component: **elliptic curves**

Author: **Frédéric Chapoton**

Branch/Commit: **[`c0eaf1e`](https://github.com/sagemath/sagetrac-mirror/commit/c0eaf1e9a7ad01bbf8c6a464afbfee438cc0c62f)**

Reviewer: **Lorenz Panny**

_Issue created by migration from https://trac.sagemath.org/ticket/34736_

