build(deps): bump petgraph from 0.6.3 to 0.6.4 (#118)

Bumps [petgraph](https://github.com/petgraph/petgraph) from 0.6.3 to 0.6.4.
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](https://github.com/petgraph/petgraph/compare/[email protected]@v0.6.4)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>