Update petgraph requirement from 0.5 to 0.6

[dependabot]

Updates the requirements on petgraph to permit the latest version.

Changelog

Sourced from petgraph's changelog.

Version 0.6.0 (2021-07-04)

Breaking changes

• MSRV is now 1.14 ([#444](https://github.com/petgraph/petgraph/issues/444)_).
• Removed the NodeCompactIndexable trait impl for MatrixGraph ([#429](https://github.com/petgraph/petgraph/issues/429)_).
• The IntoEdges::edges implementations are now required return edges with the passed node as source ([#433](https://github.com/petgraph/petgraph/issues/433)_).

New features

• Multiple documentation improvements ([#360](https://github.com/petgraph/petgraph/issues/360), [#383](https://github.com/petgraph/petgraph/issues/383), [#426](https://github.com/petgraph/petgraph/issues/426), [#433](https://github.com/petgraph/petgraph/issues/433), [#437](https://github.com/petgraph/petgraph/issues/437), [#443](https://github.com/petgraph/petgraph/issues/443), [#450](https://github.com/petgraph/petgraph/issues/450)_).
• Added an immediately_dominated_by method to the dominators result ([#337](https://github.com/petgraph/petgraph/issues/337)_).
• Added adj::List, a new append-only graph type using a simple adjacency list with no node-weights ([#263](https://github.com/petgraph/petgraph/issues/263)_).
• Added dag_to_toposorted_adjacency_list and dag_transitive_reduction_closure algorithms to transitively reduce an acyclic graph ([#263](https://github.com/petgraph/petgraph/issues/263)_).
• Made the is_isomorphic algorithm generic on both graph types ([#369](https://github.com/petgraph/petgraph/issues/369)_).
• Implement Debug and Clone for all the iterators ([#418](https://github.com/petgraph/petgraph/issues/418)_).
• Implement multiple mising traits on graph implementations and adapters ([#405](https://github.com/petgraph/petgraph/issues/405), [#429](https://github.com/petgraph/petgraph/issues/429)).
• Add an EdgeIndexable public trait ([#402](https://github.com/petgraph/petgraph/issues/402)_).
• Added immutable node_weights and edge_weights methods for Graph and StableGraph ([#363](https://github.com/petgraph/petgraph/issues/363)_).

New algorithms

• Added a k-shortest-path implementation ([#328](https://github.com/petgraph/petgraph/issues/328)_).
• Added a generic graph complement implementation ([#371](https://github.com/petgraph/petgraph/issues/371)_).
• Added a maximum matching implementation ([#400](https://github.com/petgraph/petgraph/issues/400)_).
• Added a Floyd-Warshall shortest path algorithm ([#377](https://github.com/petgraph/petgraph/issues/377)_).
• Added a greedy feedback arc set algorithm ([#386](https://github.com/petgraph/petgraph/issues/386)_).
• Added a find_negative_cycle algorithm ([#434](https://github.com/petgraph/petgraph/issues/434)_).

Performance

• Reuse the internal state in tarjan_scc ([#313](https://github.com/petgraph/petgraph/issues/313)_)
• Reduce memory usage in tarjan_scc ([#413](https://github.com/petgraph/petgraph/issues/413)_).
• Added tighter size hints to all iterators ([#380](https://github.com/petgraph/petgraph/issues/380)_).
• Optimized petgraph::dot a bit ([#424](https://github.com/petgraph/petgraph/issues/424)_).
• Optimized StableGraph de-serialization with holes ([#395](https://github.com/petgraph/petgraph/issues/395)_).

Bug fixes

• Fixed A* not producing optimal solutions with inconsistent heuristics ([#379](https://github.com/petgraph/petgraph/issues/379)_).
• Fixed a stacked borrow violation ([#404](https://github.com/petgraph/petgraph/issues/404)_).
• Fixed a panic in StableGraph::extend_with_edges ([#415](https://github.com/petgraph/petgraph/issues/415)_).
• Fixed multiple bugs in the matrix graph implementation ([#427](https://github.com/petgraph/petgraph/issues/427)_).
• Fixed GraphMap::remove_node not removing some edges ([#432](https://github.com/petgraph/petgraph/issues/432)_).

... (truncated)

Commits

• 9c832eb Release 0.6 (#452)
• d574b65 Move changelog to RELEASES.rst and add 0.6 changes (#451)
• 44142c7 Fix bare url in doc (#450)
• 9d66175 Address more clippy lints (#449)
• ec1b5bc Organize visit trait implementations (#448)
• 2bfdefa Move algorithm modules to algo/ (#447)
• 5c8b1d9 Fix minor clippy warnings (#440)
• 23f6a64 Bump MSRV to 1.41, update dependencies (#444)
• f15595b Require the index to be valid in NodeIndexable::from_index (#443)
• d1168b0 Avoid creating dummy NodeIds in matching (#442)
• Additional commits viewable in compare view

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