update contourf hatch test with cmap.with_alpha()

Author: r3kste

lib/matplotlib/tests/test_axes.py

@@ -2649,8 +2649,8 @@ def test_contour_hatching():
     x, y, z = contour_dat()
     fig, ax = plt.subplots()
     ax.contourf(x, y, z, 7, hatches=['/', '\\', '//', '-'],
-                cmap=mpl.colormaps['gray'],
-                extend='both', alpha=0.5)
+                cmap=mpl.colormaps['gray'].with_alpha(0.5),
+                extend='both')
 
 
 @image_comparison(['contour_colorbar'], style='mpl20',