Change LOG.warn to LOG.warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning
so we prefer to use warning to avoid DeprecationWarning.

Change-Id: I3b8651ee69c9c817ad47db13b06f2f92077a3923
Closes-Bug: #1530742

kingbird/db/sqlalchemy/api.py

@@ -65,8 +65,8 @@ def _session(context):
 def is_admin_context(context):
     """Indicates if the request context is an administrator."""
     if not context:
-        LOG.warn(_('Use of empty request context is deprecated'),
-                 DeprecationWarning)
+        LOG.warning(_('Use of empty request context is deprecated'),
+                    DeprecationWarning)
         raise Exception('die')
     return context.is_admin