Fix all_cycles_iterator for immutable graphs

[Ordoviz]

This small PR fixes TypeError: this graph is immutable and so cannot be changed when calling G.all_simple_cycles(), where G is a non-empty immutable graph.

See also #41487

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[dcoudert]

A better solution is to add parameter immutable to methods strongly_connected_components_subgraphs and biconnected_components_subgraphs. This way we avoid extra copies of the graphs.

For undirected graph, we can do it in #40965 if needed.

[Ordoviz]

Do you mean adding parameter immutable to biconnected_components_subgraphs and changing its implementation to [G.subgraph(c, immutable=immutable) for c in G.strongly_connected_components()] to avoid having to inline this function call here?

I'm not following how the extra parameter would help to avoid copies. The cycle finding algorithm needs each component hh to be a mutable copy. Since we also delete edges from the components, it would not be sufficient to keep track of forbidden vertices.

[dcoudert]

no, I mean to use in all_cycles_iterator a call to G.strongly_connected_components_subgraphs(immutable=False) and to modify method G.strongly_connected_components_subgraphs to return mutable/imutable subgraphs.

Your solution is fine. My proposal is more general and can be useful for other methods.

[dcoudert]

See #41493.

[dcoudert]

LGTM.

This solution can be modified later, when #41493 will be included.