Added python 3 support and updated dependencies

[maxmalysh]

Hi! I've added Python 3 support by using basestring from future package (the corresponding import is past.builtins), now it works both in Python 2 and 3.

Also I've updated ethereum and requests versions in setup.py and requirements.txt, but it is another commit, so you can merge only Python 3 stuff if you don't want to mess with dependencies.

[maurycyp]

Hi @maxmalysh , thanks for this PR. I should have it tested, merged, and released this weekend.

If I remember correctly, the main thing that was holding back Python3 support was the ethereum library. It's now at 1.6.0 (from 1.5.2), and apparently supports Python3.

future is at 0.16.0 (from 0.15.2), so that needs to change as well.

I'm also thinking of unpinning the version of requests, since their API and release process is quite good. What do you think?

[tdickman]

Can we merge this?