Add py36 testenv

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
Therefore, according to Transition Plan [1],
it'll be handy to have py36 testenv.

For more details, please check Python2 Deprecation Timeline [2]
and Python3-first Goal - Completion Criteria [3].

[1] https://wiki.ubuntu.com/Python/Python36Transition
[2] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html
[3] https://review.openstack.org/#/c/575933/8/goals/stein/python3-first.rst@42

Change-Id: I4839bb3c9905a540e3ed61a36cacbf9a5f665b1b

tox.ini

@@ -57,6 +57,17 @@ commands =
     rm -f .coverage
     rm -rf .testrepository
 
+[testenv:py36]
+basepython = python3.6
+setenv =
+  OS_TEST_PATH = ./freezer/tests/unit
+commands =
+    find . -type f -name "*.pyc" -delete
+    python setup.py testr --coverage --testr-args="{posargs}"
+    coverage report -m
+    rm -f .coverage
+    rm -rf .testrepository
+
 [testenv:docs]
 whitelist_externals = rm
                       bash