Add py36 testenv

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
Therefore, according to transition plan [1],
we should have py36 testenv ready for it.

[1] https://wiki.ubuntu.com/Python/Python36Transition

Change-Id: I9bc6d827e66a57df3311c40d054faaa58533b811

tox.ini

@@ -45,6 +45,17 @@ commands =
     rm -f .coverage
     rm -rf .testrepository
 
+[testenv:py36]
+basepython = python3.6
+setenv =
+  OS_TEST_PATH = ./freezerclient/tests/unit
+commands =
+    find . -type f -name "*.py[c|o]" -delete
+    python setup.py testr --coverage --testr-args="{posargs}"
+    coverage report -m
+    rm -f .coverage
+    rm -rf .testrepository
+
 [testenv:docs]
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -b html doc/source doc/build/html