Trac #21345: very minor speedup in edge_labels

by creating the list directly

URL: https://trac.sagemath.org/21345
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): David Coudert

src/sage/graphs/generic_graph.py

@@ -10813,7 +10813,7 @@ def edge_label(self, u, v=None):
 
     def edge_labels(self):
         """
-        Returns a list of edge labels.
+        Return a list of edge labels.
 
         EXAMPLES::
 
@@ -10824,10 +10824,7 @@ def edge_labels(self):
             sage: G.edge_labels()
             ['x', 'z', 'a', 'out']
         """
-        labels = []
-        for u,v,l in self.edges():
-            labels.append(l)
-        return labels
+        return [l for _, _, l in self.edge_iterator()]
 
     def remove_multiple_edges(self):
         """