Merge pull request matplotlib#13393 from anntzer/is_frame_like

Deprecate Spine.is_frame_like.

Author: QuLogic

doc/api/next_api_changes/2019-02-08-AL.rst

@@ -0,0 +1,5 @@
+Deprecations
+````````````
+
+``Spine.is_frame_like`` is deprecated.  It has never been used in the codebase
+since its addition in 2009.

lib/matplotlib/spines.py

@@ -177,6 +177,7 @@ def cla(self):
         if self.axis is not None:
             self.axis.cla()
 
+    @cbook.deprecated("3.1")
     def is_frame_like(self):
         """return True if directly on axes frame