remove six dependencies (#213)

Author: glemaitre

imblearn/base.py

@@ -8,7 +8,6 @@
 from collections import Counter
 
 import numpy as np
-from six import string_types
 from sklearn.base import BaseEstimator
 from sklearn.externals import six
 from sklearn.utils import check_X_y
@@ -204,7 +203,7 @@ def _validate_ratio(self):
             elif self.ratio <= 0:
                 raise ValueError('Ratio cannot be negative.')
 
-        elif isinstance(self.ratio, string_types):
+        elif isinstance(self.ratio, six.string_types):
             if self.ratio != 'auto':
                 raise ValueError('Unknown string for the parameter ratio.')
         else:

imblearn/under_sampling/instance_hardness_threshold.py

@@ -6,10 +6,10 @@
 from collections import Counter
 
 import numpy as np
-from six import string_types
 import sklearn
 from sklearn.base import ClassifierMixin
 from sklearn.ensemble import RandomForestClassifier
+from sklearn.externals.six import string_types
 
 from ..base import BaseBinarySampler