clean spanning_tree.pyx

[dcoudert]

Done in this ticket:

• rewrite/simply kruskal using proper DisjointSet
• avoid comparison of vertex labels in boruvka using frozenset
• PEP8 cleaning

Some sort operations remain in kruskal.

• Edges must be visited by increasing weights. However, there is no reason for sorting by vertex labels as well
• the output list of edges is sorted. There is no reason for this, especially since other methods don't do that, e.g., boruvka

So we can decide to change that as well in this ticket.

[dcoudert]

New commits:

​dce10d8

trac #26547: clean spanning_tree.pyx

[tscrim]

LGTM.