cleanup and code details in cluster_algebra_quiver

[fchapoton]

some changes as suggested by pycodestyle and ruff

also some further minor code enhancements,

and shortening one long time doctest

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[dcoudert]

in file mutation_type.py, several elif could be turned to if and some else could be removed, in particular in method _connected_mutation_type

[fchapoton]

Oui, ce code est asse alambiqué. On peut garder ca pour une autre fois ?

[dcoudert]

oui, pas de souci.

[dcoudert]

LGTM.