MAINT: Use Python 3 compatible metaclass.

Use six's with_metaclass to have objects that use metaclasses, in
both Python 2 and 3.

Otherwise, in Python 3 the objects were being treated as if they
did not have a metaclass, when the Python 2 syntax is used, leading
to errors because of missing attributes, etc.

zipline/sources/data_source.py

@@ -3,13 +3,13 @@ from abc import (
     abstractproperty
 )
 
+from six import with_metaclass
+
 from zipline.protocol import DATASOURCE_TYPE
 from zipline.protocol import Event
 
 
-class DataSource(object):
-
-    __metaclass__ = ABCMeta
+class DataSource(with_metaclass(ABCMeta)):
 
     @property
     def event_type(self):