pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 9)

<div id="comment:0"></div>

Continue the cleaning of generic_graph.py after #34396.

Depends on #34396

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`bfbb3cf`](https://github.com/sagemath/sagetrac-mirror/commit/bfbb3cf43fb87058a20bb9b8ee29d27a23241559)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34397_

