correct string checking

Author: janvanrijn

openml/datasets/dataset.py

@@ -2,6 +2,7 @@
 import io
 import logging
 import os
+import six
 import sys
 
 import arff
@@ -10,7 +11,6 @@
 import scipy.sparse
 import xmltodict
 
-from builtins import str
 from .data_feature import OpenMLDataFeature
 from ..exceptions import PyOpenMLError
 
@@ -66,7 +66,7 @@ def __init__(self, dataset_id=None, name=None, version=None, description=None,
         self.default_target_attribute = default_target_attribute
         self.row_id_attribute = row_id_attribute
         self.ignore_attributes = None
-        if isinstance(ignore_attribute, str):
+        if isinstance(ignore_attribute, six.string_types):
             self.ignore_attributes = [ignore_attribute]
         elif isinstance(ignore_attribute, list):
             self.ignore_attributes = ignore_attribute