Adds future dependency. - #744

I am not sure do we need this in the requirements file. What problem you are trying to solve with this PR?

The future (or futurize) package is to have python 2.7 and 3.x compatibility. There are some methods useful in python 3.x, like the with_metaclass, there are types definitions to have simultaneous compatibility. The first important code it brought was the print() function to python 2.6. We can use future, or we can get only the methods we need from it (the files requiring it are just a few).

I thought we already discussed what we specifically don't want to use future, six or any other such module at least as an extra external dependency. I doubt we in the end need them at all, but if we do, we can either vendor a whole module or just copy functions we need. For example, with_metaclass is just an few lines of (very interesting) code.