polygonal circuit tests: added missing import

https://travis-ci.org/fphammerle/freesurfer-surface/jobs/534271091

tests/test_polygonal_circuit.py

@@ -1,6 +1,6 @@
 import pytest
 
-from freesurfer_surface import PolygonalCircuit
+from freesurfer_surface import PolygonalCircuit, Vertex
 
 
 def test_init_tuple():