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.
So that algorithms that specifiy market_aware, days and delta
as args can be transitioned to just specifying a window_length kwarg.
Moving towards removing market_aware and delta completely.
Eddie Hebert