build(deps): update petgraph requirement from ^0.6 to ^0.7 (#1145)

Updates the requirements on [petgraph](https://github.com/petgraph/petgraph) to permit the latest version.
- [Changelog](https://github.com/petgraph/petgraph/blob/master/RELEASES.rst)
- [Commits](https://github.com/petgraph/petgraph/compare/petgraph@v0.6.0...petgraph@v0.7.1)

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

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Author: dependabot[bot]

Cargo.toml

@@ -18,7 +18,7 @@ num = "^0.4"
 num-derive = "^0.4"
 num-format = "^0.4"
 num-traits = "^0.2"
-petgraph = "^0.6"
+petgraph = "^0.7"
 regex = "^1.7"
 serde = { version = "^1.0", features = ["derive"] }
 termcolor = "^1.2"