gh-35551: Enable access to some methods in sage/graphs/graph_decompositions
    
### 📚 Description

Files `src/sage/graphs/graph_decompositions/bandwidth.pyx` and
`src/sage/graphs/graph_decompositions/cutwidth.pyx` were missing the
statement `cython: binding=True`. So these methods were hardly
accessible. We add the instruction and provide access in graphs.

### 📝 Checklist

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- [x] The title is concise, informative, and self-explanatory.
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- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies
    
URL: #35551
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/graphs/graph.py

@@ -9987,6 +9987,8 @@ def bipartite_double(self, extended=False):
     from sage.graphs.graph_decompositions.vertex_separation import pathwidth
     from sage.graphs.graph_decompositions.tree_decomposition import treelength
     from sage.graphs.graph_decompositions.clique_separators import atoms_and_clique_separators
+    from sage.graphs.graph_decompositions.bandwidth import bandwidth
+    from sage.graphs.graph_decompositions.cutwidth import cutwidth
     from sage.graphs.matchpoly import matching_polynomial
     from sage.graphs.cliquer import all_max_clique as cliques_maximum
     from sage.graphs.cliquer import all_cliques

src/sage/graphs/graph_decompositions/bandwidth.pyx

@@ -1,3 +1,4 @@
+# cython: binding=True
 r"""
 Bandwidth of undirected graphs
 

src/sage/graphs/graph_decompositions/cutwidth.pyx

@@ -1,3 +1,4 @@
+# cython: binding=True
 r"""
 Cutwidth