add parameter immutable to transitive closure methods in sage/graphs/generic_graph.py

[dcoudert]

Following #39280 and discussions in #39177, we add parameter immutable to methods related to transitive closure in sage/graphs/generic_graph.py and sage/graphs/generic_graph_pyx.pyx:

• transitive_closure. We also fix the use of parameter loops that was previously ignored
• transitive_reduction
• transitive_reduction_acyclic
• is_transitively_reduced. Here we ensure that the method accepts immutable digraphs.

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