Try changing the test to import cartopy

tests/test_velocity_plot_util.py

@@ -118,18 +118,20 @@ def test_velocity_grid_to_geo():
 
 def test_plot_polar_contour():
     """Test the plot_util.plot_polar_contour function."""
-    lat = np.array(([50, 48, 46], [60, 58, 56], [70, 68, 66]))  # y, x
-    lon = np.array(([5, 8, 11], [6, 9, 12], [7, 10, 13]))
-    temp = np.array(([2, 1, 0], [2, 1, 0], [2, 2, 1]))
-    figsize = (5, 5)  # Figure size
     try:
+        import cartopy.crs as ccrs  # mapping plots
+        lat = np.array(([50, 48, 46], [60, 58, 56], [70, 68, 66]))  # y, x
+        lon = np.array(([5, 8, 11], [6, 9, 12], [7, 10, 13]))
+        temp = np.array(([2, 1, 0], [2, 1, 0], [2, 2, 1]))
+        figsize = (5, 5)  # Figure size
+
         mrc = ccrs.NorthPolarStereo(central_longitude=0.0)
         fig = plt.figure(figsize=figsize)
         ax1 = fig.add_axes([0.1, 0.1, 0.8, 0.75], projection=mrc)
         cs1 = plot_util.plot_polar_contour(lon, lat, temp, ax1)
         assert isinstance(cs1, cartopy.mpl.contour.GeoContourSet)
-    except AssertionError:
-        assert False
+    except ImportError:
+        warn("No cartopy found - please run\nconda install -c conda-forge cartopy")
 
 
 def test_set_circle():