Trac #34309: pycodestyle cleanup in src/sage/graphs/generators/famili…

…es.py

We clean all issues raised by pycodestyle except `E741 ambiguous
variable name 'l'`.

URL: https://trac.sagemath.org/34309
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Frédéric Chapoton