Enable Python 3 tests by default in Tox config

Now that we're supporting it, I plan to run the Python 3 tests as part of
my normal coding workflow.

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27-test, py27-flake8, docs
+envlist = py27-test, py35-test, py27-flake8, docs
 
 # The exhaustive list of environments is:
 # envlist = py{27,34,35}-{test,cov}, py{27,34,35}-flake8, docs