gh-38535: fix issue 38527 with `spqr_tree_to_graph`
Fixes #38527.
This PR makes the comparison of edges more robust in
`spqr_tree_to_graph`, so that edges `(u, v, label)` and `(v, u, label)`
are no longer considered as distinct edges.
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38535
Reported by: David Coudert
Reviewer(s): cyrilbouvier
