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: Ie8b7a9e205f0276b20b86a4dffd51bcaccb67842

src/bin/pegleg/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, pep8
+envlist = py35, py36, pep8
 skipsdist = True
 
 [testenv]

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, pep8, docs
+envlist = py35, py36, pep8, docs
 # Allows docs to be built without setup.py having to exist. Requires that
 # usedevelop be False as well (which it is by default).
 skipsdist = True
@@ -16,6 +16,12 @@ commands =
     tox -c src/bin/pegleg/tox.ini -e py35
 whitelist_externals = tox
 
+[testenv:py36]
+commands =
+    # Run all unit tests under src/bin/pegleg
+    tox -c src/bin/pegleg/tox.ini -e py36
+whitelist_externals = tox
+
 [testenv:fmt]
 commands =
     tox -c src/bin/pegleg/tox.ini -e fmt