Update PyPlot.py (petercorke#403)

w_xaxis are deprecated (https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.6.0.html)

Author: renaudponcelet

roboticstoolbox/backends/PyPlot/PyPlot.py

@@ -416,9 +416,9 @@ def getframe(self):
                 )
 
         # make the background white, looks better than grey stipple
-        self.ax.w_xaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
-        self.ax.w_yaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
-        self.ax.w_zaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
+        self.ax.xaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
+        self.ax.yaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
+        self.ax.zaxis.set_pane_color((1.0, 1.0, 1.0, 1.0))
         plt.gcf().canvas.draw()
 
         # render the frame and save as a PIL image in the list