Don't pickle Axes' cached renderer.

It's not allowed to be pickled on a Figure, so presumably it shouldn't
be allowed on the Axes either.

Fixes matplotlib#6870.
Fixes matplotlib#6181.
Fixes matplotlib#3899.

Author: QuLogic

lib/matplotlib/axes/_base.py

@@ -582,6 +582,13 @@ def __init__(self, fig, rect,
             right=rcParams['ytick.right'] and rcParams['ytick.major.right'],
             which='major')
 
+    def __getstate__(self):
+        # The renderer should be re-created by the figure, and then cached at
+        # that point.
+        state = super(_AxesBase, self).__getstate__()
+        state['_cachedRenderer'] = None
+        return state
+
     def __setstate__(self, state):
         self.__dict__ = state
         # put the _remove_method back on all artists contained within the axes