pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 4)

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Depends on #34357

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`b1db4f1`](https://github.com/sagemath/sagetrac-mirror/commit/b1db4f146caea0add146573237468ee6a88bb7d5)**

Reviewer: **Kwankyu Lee**

_Issue created by migration from https://trac.sagemath.org/ticket/34358_

