Update petgraph requirement from 0.6.0 to 0.7.1

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
...

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

Cargo.toml

@@ -17,7 +17,7 @@ include = ["src/**/*", "LICENSE-*", "README.*", "CHANGELOG.md"]
 maintenance = {status = "passively-maintained"}
 
 [dependencies]
-petgraph = "0.6.0"
+petgraph = "0.7.1"
 xml-rs = "0.8.0"
 
 [dev-dependencies]