resolve deprecation warnings from matplotlib in docs scripts

doc/source/cookbook/streamlines.py

@@ -33,7 +33,9 @@
 
 # Create a 3D plot, trace the streamlines through the 3D volume of the plot
 fig = plt.figure()
-ax = Axes3D(fig)
+ax = Axes3D(fig, auto_add_to_figure=False)
+fig.add_axes(ax)
+
 for stream in streamlines.streamlines:
     stream = stream[np.all(stream != 0.0, axis=1)]
     ax.plot3D(stream[:, 0], stream[:, 1], stream[:, 2], alpha=0.1)

doc/source/cookbook/streamlines_isocontour.py

@@ -32,7 +32,8 @@
 
 # Create a 3D matplotlib figure for visualizing the streamlines
 fig = plt.figure()
-ax = Axes3D(fig)
+ax = Axes3D(fig, auto_add_to_figure=False)
+fig.add_axes(ax)
 
 # Trace the streamlines through the volume of the 3D figure
 for stream in streamlines.streamlines:

doc/source/cookbook/surface_plot.py

@@ -20,7 +20,7 @@
 
 # Create a 3D matplotlib figure for visualizing the surface
 fig = plt.figure()
-ax = fig.gca(projection="3d")
+ax = fig.add_subplot(projection="3d")
 p3dc = Poly3DCollection(surface.triangles, linewidth=0.0)
 
 # Set the surface colors in the right scaling [0,1]