Switch to the python:3.6 image for CI, to make sure we get Python 3.6

.gitlab-ci.yml

@@ -1,7 +1,7 @@
-test:
+image: python:3.6
-  script:
+before_script:
-    - apt-get update -qy
-    - apt-get install -y python-dev python-pip
   - pip install pipenv
   - pipenv install
+test:
+  script:
     - pipenv run pytest