Use basestring instead of str, like in FluentHandler._add_dic

For consistency's sake.

Author: EvaSDK

fluent/handler.py

@@ -47,7 +47,7 @@ def format(self, record):
     def _structuring(self, data, msg):
         if isinstance(msg, dict):
             self._add_dic(data, msg)
-        elif isinstance(msg, str):
+        elif isinstance(msg, basestring):
             try:
                 self._add_dic(data, json.loads(str(msg)))
             except (ValueError, json.JSONDecodeError):