Fix #13711 by importing cbook.

efiring · efiring · commit c71395c62f01 · 2019-05-22T11:51:12.000-10:00
diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -11,7 +11,7 @@
 import numpy as np
 
 import matplotlib
-from matplotlib import backend_tools, rcParams
+from matplotlib import backend_tools, rcParams, cbook
 from matplotlib.backend_bases import (
     _Backend, FigureCanvasBase, FigureManagerBase, NavigationToolbar2,
     StatusbarBase, TimerBase, ToolContainerBase, cursors)
