feat(py3/py27): integrate pytest-html

In addition to gitlab integration, this one provides quite additional feature:

• display captured stdout/stderr which can been needed for debugging
• is present for everyjob while gitlab integration is only for the MR
• I find the output more useful (because more information) and easier to use that gitlab no so great UI (but less easy to reach)
• it's pretty much free to add

templates/tests/py27.yml

 ---
 py27:
   before_script:
-    - pip install tox
+    - pip install tox pytest-html
   stage: tests
-  script: tox -e py27
+  script: tox -e py27 -- --html=py3.html --self-contained-html
   artifacts:
     paths:
+      - py27.html
       - py27-deprecated-warnings.json