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/transforms/stddev.py

@@ -16,19 +16,20 @@
 from collections import defaultdict
 from math import sqrt
 
+from six import with_metaclass
+
 from zipline.errors import WrongDataForTransform
 from zipline.transforms.utils import EventWindow, TransformMeta
 import zipline.utils.math_utils as zp_math
 
 
-class MovingStandardDev(object):
+class MovingStandardDev(with_metaclass(TransformMeta)):
     """
     Class that maintains a dictionary from sids to
     MovingStandardDevWindows.  For each sid, we maintain a the
     standard deviation of all events falling within the specified
     window.
     """
-    __metaclass__ = TransformMeta
 
     def __init__(self, market_aware=True, window_length=None, delta=None):