Use py36

This commit makes to use python3.6 instead of python2.7 in postinstall
since python2.7 will be EOL soon. And, this commit encourages to use
py36 task instead of py27 as well.
And also this commit changes to use node8 to test in the upstream gate.

Change-Id: I75f0045e88d20d04ecbcdc798bcf3f45d2c27e7a

.zuul.yaml

@@ -8,7 +8,7 @@
 
 - project:
     templates:
-      - nodejs4-jobs
+      - nodejs8-jobs
       - openstack-python-jobs
       - publish-openstack-docs-pti
       - openstack-python36-jobs

README.rst

@@ -96,7 +96,7 @@ Data should be written to :code:`stackviz-html/data/` using
 
 Testing
 =======
-* Python tests: :code:`tox -e py27`
+* Python tests: :code:`tox -e py36`
 * JavaScript unit tests: :code:`gulp unit`
 * JavaScript E2E tests: :code:`gulp e2e`
 

package.json

@@ -69,7 +69,7 @@
     "watchify": "3.7.0"
   },
   "scripts": {
-    "postinstall": "if [ ! -d .venv ]; then tox -epy27 --notest; fi",
+    "postinstall": "if [ ! -d .venv ]; then tox -epy36 --notest; fi",
     "test": "gulp unit",
     "lint": "eslint ./",
     "prod": "gulp prod; python setup.py sdist"