pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 8)

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Continue the cleaning of generic_graph.py after #34395.


Depends on #34395

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`1893699`](https://github.com/sagemath/sagetrac-mirror/commit/1893699990cc6b60ddb538c522433e57b7968595)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34396_

