pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 5)

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Continue the cleaning of `generic_graph.py` after #34358.

Depends on #34358

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`176e391`](https://github.com/sagemath/sagetrac-mirror/commit/176e3914b277ef1ff3ae678bbbca67e3ff8a5026)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34392_

