Bump petgraph from 0.5.1 to 0.6.0

Bumps [petgraph](https://github.com/petgraph/petgraph) from 0.5.1 to 0.6.0.
- [Release notes](https://github.com/petgraph/petgraph/releases)
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](petgraph/petgraph@0.5.1...0.6.0)

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

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

Author: dependabot[bot]

Cargo.lock

@@ -23,7 +23,7 @@ num-format = "^0.4"
 num = "^0.4"
 num-derive = "^0.3"
 num-traits = "^0.2"
-petgraph = "^0.5"
+petgraph = "^0.6"
 phf = { version = "^0.8", features = ["macros"] }
 regex = "^1.5"
 serde = { version = "^1.0", features = ["derive"] }