pycodestyle cleanup in src/sage/graphs/generic_graph.py (part 7)

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Continue the cleaning of generic_graph.py after #34394.

Depends on #34394

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`64abcfd`](https://github.com/sagemath/sagetrac-mirror/commit/64abcfd7bfca1509bbb0d1875303777c12258d71)**

Reviewer: **Kwankyu Lee**

_Issue created by migration from https://trac.sagemath.org/ticket/34395_

