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: Ife9ad9648ded0d96523957381f87c059c217b28b

src/bin/shipyard_airflow/tox.ini

@@ -1,10 +1,9 @@
 [tox]
-envlist = py35, pep8, bandit
+envlist = py35, py36, pep8, bandit
 
 [testenv]
 setenv=
   PYTHONWARNING=all
-basepython=python3.5
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
@@ -19,7 +18,19 @@ commands =
     --cov=shipyard_airflow \
     --cov-report=html
 
+[testenv:py36]
+skipsdist=True
+commands =
+  pytest \
+    {posargs} \
+    --cov-branch \
+    --cov-report=term-missing:skip-covered \
+    --cov-config=.coveragerc \
+    --cov=shipyard_airflow \
+    --cov-report=html
+
 [testenv:pep8]
+basepython=python3
 commands = flake8 {posargs}
 
 [testenv:bandit]

src/bin/shipyard_client/tox.ini

@@ -1,11 +1,10 @@
 [tox]
 skipsdist=True
-envlist = py35, pep8, bandit
+envlist = py35, py36, pep8, bandit
 
 [testenv]
 setenv=
   PYTHONWARNING=all
-basepython=python3.5
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
@@ -19,7 +18,18 @@ commands =
     --cov=shipyard_client \
     --cov-report=html
 
+[testenv:py36]
+commands =
+  pytest \
+    {posargs} \
+    --cov-branch \
+    --cov-report=term-missing:skip-covered \
+    --cov-config=.coveragerc \
+    --cov=shipyard_client \
+    --cov-report=html
+
 [testenv:pep8]
+basepython=python3
 commands = flake8 {posargs}
 
 [testenv:bandit]

tox.ini

@@ -6,7 +6,6 @@ envlist = all_jobs, docs
 setenv=
   PYTHONWARNING=all
 passenv=HTTP_PROXY HTTPS_PROXY http_proxy https_proxy NO_PROXY no_proxy
-basepython=python3.5
 
 # Please use `make docs` instead
 [testenv:docs]
@@ -38,6 +37,13 @@ commands=
   tox -c {toxinidir}/src/bin/shipyard_airflow/tox.ini -e py35
   tox -c {toxinidir}/src/bin/shipyard_client/tox.ini -e py35
 
+[testenv:py36]
+whitelist_externals=
+    tox
+commands=
+  tox -c {toxinidir}/src/bin/shipyard_airflow/tox.ini -e py36
+  tox -c {toxinidir}/src/bin/shipyard_client/tox.ini -e py36
+
 [testenv:pep8]
 whitelist_externals=
     tox