pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 6)

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Continue the cleaning of generic_graph.py after #34392.

Depends on #34392

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`0912bd0`](https://github.com/sagemath/sagetrac-mirror/commit/0912bd041fdf28e80f9b47115ef72433518a96c6)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34394_

