MNT: put back _hold as read-only attribute on AxesBase

This is to un-break the current version of cartopy.

Author: tacaswell

lib/matplotlib/axes/_base.py

@@ -399,6 +399,10 @@ class _AxesBase(martist.Artist):
     _shared_y_axes = cbook.Grouper()
     _twinned_axes = cbook.Grouper()
 
+    @property
+    def _hold(self):
+        return True
+
     def __str__(self):
         return "{0}({1[0]:g},{1[1]:g};{1[2]:g}x{1[3]:g})".format(
             type(self).__name__, self._position.bounds)