Add a py35 environment to tox

Infra will start running python35 jobs soon [1], targeting to replace
the current python34 jobs. This is due to the change from Ubuntu Trusty
to Xenial, where only python3.5 will be available.

So we add a py35 environment with the same definitions for skipping
not-yet-working checks as we have in place for py34 already.

[1] https://review.openstack.org/336272

Change-Id: I017478fafa31121f40c95d73e49ac4dced80c7a2

tox.ini

@@ -37,6 +37,12 @@ commands =
   find . -type f -name "*.pyc" -delete
   bash tools/pretty_tox3.sh '{posargs}'
 
+[testenv:py35]
+setenv = {[testenv]setenv}
+commands =
+  find . -type f -name "*.pyc" -delete
+  bash tools/pretty_tox3.sh '{posargs}'
+
 [testenv:functional]
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}