Bump petgraph from 0.6.5 to 0.6.6

Bumps [petgraph](https://github.com/petgraph/petgraph) from 0.6.5 to 0.6.6.
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](https://github.com/petgraph/petgraph/compare/[email protected]@v0.6.6)

---
updated-dependencies:
- dependency-name: petgraph
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>