fix issue with immutable graphs in src/sage/graphs/chrompoly.pyx

[dcoudert]

Currently, method chromatic_polynomial raises an error for immutable graphs with multiple edges.

sage: G = Graph([(0, 1), (0, 1)], multiedges=True, immutable=True)
sage: G.chromatic_polynomial()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[13], line 1
----> 1 G.chromatic_polynomial()

File ~/sage/src/sage/graphs/chrompoly.pyx:172, in sage.graphs.chrompoly.chromatic_polynomial()
    170 cdef mpz_t *coeffs
    171 G = G.relabel(inplace=False)
--> 172 G.remove_multiple_edges()
    173 G.remove_loops()
    174 nverts = G.num_verts()

File ~/sage/src/sage/graphs/generic_graph.py:13733, in GenericGraph.remove_multiple_edges(self)
  13731 labels = self.edge_label(u, v)
  13732 if len(labels) > 1:
> 13733     self.delete_edges((u, v, labels[i]) for i in range(1, len(labels)))

File ~/sage/src/sage/graphs/generic_graph.py:12843, in GenericGraph.delete_edges(self, edges)
  12819 def delete_edges(self, edges):
  12820     """
  12821     Delete edges from an iterable container.
  12822 
   (...)
  12841         120
  12842     """
> 12843     self._backend.del_edges(edges, self._directed)

File ~/sage/src/sage/graphs/base/static_sparse_backend.pyx:707, in sage.graphs.base.static_sparse_backend.StaticSparseBackend.del_edges()
    705         ValueError: graph is immutable; please change a copy instead (use function copy())
    706     """
--> 707     raise ValueError("graph is immutable; please change a copy instead (use function copy())")
    708 
    709 def set_edge_label(self, u, v, l, directed):

ValueError: graph is immutable; please change a copy instead (use function copy())

This is due to a call to relabel followed by a call to remove_multiple_edges which fails in this case.

When algorithm='Python', method chromatic_polynomial_with_cache needs a mutable copy of the input graph.

We fix both issue. Further more, we avoid trying to remove loops from the graph as the chromatic polynomial of a graph with loops is 0.

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