Move toolmanager warning from logging to warning.

anntzer · anntzer · commit 1b5fa400bd7c · 2018-11-05T17:21:25.000+01:00
Using warnings.warn avoids emitting the warning once per Tool, i.e. a
dozen times at each invocation.  (Yes, the system is experimental, we
got that part.)

Also update the warning message (Matplotlib 2.1 is long past).
diff --git a/lib/matplotlib/backend_tools.py b/lib/matplotlib/backend_tools.py
@@ -77,9 +77,9 @@ class ToolBase(object):
     """
 
     def __init__(self, toolmanager, name):
-        _log.warning('Treat the new Tool classes introduced in v1.5 as '
-                     'experimental for now, the API will likely change in '
-                     'version 2.1, and some tools might change name')
+        cbook._warn_external(
+            'The new Tool classes introduced in v1.5 are experimental; their '
+            'API (including names) will likely change in future versions.')
         self._name = name
         self._toolmanager = toolmanager
         self._figure = None
