Update petgraph requirement from 0.5 to 0.6 (#64)
Update petgraph requirement from 0.5 to 0.6 (#64)

Updates the requirements on [petgraph](https://github.com/petgraph/petgraph) to permit the latest version.
- [Release notes](https://github.com/petgraph/petgraph/releases)
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](https://github.com/petgraph/petgraph/compare/0.5.0...0.6.0)

---
updated-dependencies:
- dependency-name: petgraph
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
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