Update petgraph requirement from 0.4 to 0.6

[dependabot-preview]

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).
• Removed the NodeCompactIndexable trait impl for MatrixGraph (#429).
• The IntoEdges::edges implementations are now required return edges with the passed node as source (#433).

New features

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

New algorithms

• Added a k-shortest-path implementation (#328).
• Added a generic graph complement implementation (#371).
• Added a maximum matching implementation (#400).
• Added a Floyd-Warshall shortest path algorithm (#377).
• Added a greedy feedback arc set algorithm (#386).
• Added a find_negative_cycle algorithm (#434).

Performance

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

Bug fixes

• Fixed A* not producing optimal solutions with inconsistent heuristics (#379).
• Fixed a stacked borrow violation (#404).
• Fixed a panic in StableGraph::extend_with_edges (#415).
• Fixed multiple bugs in the matrix graph implementation (#427).
• Fixed GraphMap::remove_node not removing some edges (#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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