fix some ruff suggestions in cluster_algebra_quiver

[fchapoton]

this was scripted using ruff for the codes UP and C4 in the cluster folder.

Plus a few hand-made changes about pep8

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.

[github-actions]

Documentation preview for this PR (built with commit 6633a9f; changes) is ready! 🎉

[tscrim]

LGTM.

I will say one comment: I don't really care for the change set(...) to {...}. The latter is very close to a dict construction in Python, leading to a very easy to make error of {} meant to be the empty set.