sagemathgh-39896: fix issue with immutable graphs in `src/sage/graphs/chrompoly.pyx`
    
Currently, method `chromatic_polynomial` raises an error for immutable
graphs with multiple edges.
```sage
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.


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39896
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39896: fix issue with immutable graphs in `src/sage/graphs/chrompoly.pyx`
    
Currently, method `chromatic_polynomial` raises an error for immutable
graphs with multiple edges.
```sage
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.


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39896
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39896: fix issue with immutable graphs in `src/sage/graphs/chrompoly.pyx`
    
Currently, method `chromatic_polynomial` raises an error for immutable
graphs with multiple edges.
```sage
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.


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39896
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton