gh-36728: cython-lint for graphs/
fix many small details in pyx files in graphs, as found by
`cython-lint --ignore=E501,E741,E265,E221,E231,E201,E127,E129
src/sage/graphs/`
notably removing many unused def and variables.
There remains in particular a rather suspicious warning:
`src/sage/graphs/distances_all_pairs.pyx:1977:9: 'ecc_antipode' defined
but unused (try prefixing with underscore?)`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36728
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
