build(deps): update petgraph requirement from ^0.7 to ^0.8

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/CHANGELOG.md)
- [Commits](https://github.com/petgraph/petgraph/compare/[email protected]@v0.8.1)

---
updated-dependencies:
- dependency-name: petgraph
  dependency-version: 0.8.1
  dependency-type: direct:production
...

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

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 = "^0.7"
+petgraph = "^0.8"
 regex = "^1.7"
 serde = { version = "^1.0", features = ["derive"] }
 termcolor = "^1.2"