Avoid calling vertices() in graph_isom_equivalent_non_edge_labeled_graph()

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CC:  @dcoudert

Component: **graph theory**

Author: **Jeroen Demeyer**

Branch/Commit: **[`6cfcbb0`](https://github.com/sagemath/sagetrac-mirror/commit/6cfcbb0d79e08eafea87f9a29f7fe7a0e4717fb3)**

Reviewer: **Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/27029_

