gh-35551: Enable access to some methods in sage/graphs/graph_decompositions
### :books: 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.
### :memo: Checklist
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- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
URL: https://github.com/sagemath/sage/pull/35551
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton
