Start support for py3.6.

Looks like there are no problems with Python 3.6 yet. We make it a
regular target in tox and will start official support in `setup.py`
after a while.

tox.ini

@@ -3,7 +3,7 @@ addopts = --doctest-modules --doctest-glob='*.rst'
           diceware tests docs README.rst
 
 [tox]
-envlist = flake8, pypy, py26, py27, py33, py34, py35, coverage
+envlist = flake8, pypy, py26, py27, py33, py34, py35, py36, coverage
 
 [testenv]
 usedevelop=True