bug in picos.sum

picos.sum somtimes fail to iterate over dictionary of variables. Here is a minimal example:

import picos

x = {(0,1,3): picos.RealVariable('x[0,1,3]',1),
     (0,2,1): picos.RealVariable('x[0,2,1]',1),
     (1,1,1): picos.RealVariable('x[1,1,1]',1)}

sumx = picos.sum([x[key] for key in x])

This works with python3, but raises an AssertionError under python2. I know we do not want to support picos2 anymore, but if this is just a small fix, it could be nice to fix this in the next release.