client: use six.string_types instead of basestring

Which is gone on modern python.

Author: xrmx

splunklib/client.py

@@ -3591,7 +3591,7 @@ def update_index(self, name, value):
         :return: Result of POST request
         """
         kwargs = {}
-        kwargs['index.' + name] = value if isinstance(value, basestring) else json.dumps(value)
+        kwargs['index.' + name] = value if isinstance(value, six.string_types) else json.dumps(value)
         return self.post(**kwargs)
 
     def update_field(self, name, value):