Correct unicode string for python2

Irina Nicolae · Irina Nicolae · commit dadd09356d31 · 2018-05-03T11:43:34.000+01:00
diff --git a/art/classifiers/classifier.py b/art/classifiers/classifier.py
@@ -7,7 +7,7 @@
 if sys.version_info >= (3, 4):
     ABC = abc.ABC
 else:
-    ABC = abc.ABCMeta('ABC', (), {})
+    ABC = abc.ABCMeta(str('ABC'), (), {})
 
 
 class Classifier(ABC):
