clean one file about N=2 super-conformal Lie algebra

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full pep8

Component: **algebra**

Author: **Frédéric Chapoton**

Branch/Commit: **[`d1b282e`](https://github.com/sagemath/sagetrac-mirror/commit/d1b282e09c49bd26e9cce8b9c5f2c280e6cb2cfb)**

Reviewer: **David Ayotte**

_Issue created by migration from https://trac.sagemath.org/ticket/34555_

