ci(.gitlab-ci/tox): switch tox to generic python 3 and add matching job in .gitlab-ci.yaml

3 jobs for topic/default/py34-py35-gitlab-ci in 1 minute and 15 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Test
passed flake8 #61948

00:00:39

passed py27 #61949

00:01:15

passed py3 #61950

00:01:01