tox: Use Python 3.6 in default tests
Just typing `tox` should use the most relevant Python versions.
# and then run "tox" from this directory.
[tox]
-envlist = py27-test, py35-test, py27-flake8, docs
+envlist = py27-test, py36-test, py27-flake8, docs
# The exhaustive list of environments is:
# envlist = py{27,34,35}-{test,cov}, py{27,34,35}-flake8, docs
