add python version-specific test stages to CI tasks

.gitlab-ci.yml

@@ -20,6 +20,27 @@ test-centos7:
   - pip3.5 install mod_wsgi
   - python setup.py test
 
+test-python35:
+  image: python:3.5
+  stage: test
+  script:
+  - pip install -r requirements.txt
+  - python manage.py test
+
+test-python36:
+  image: python:3.6
+  stage: test
+  script:
+  - pip install -r requirements.txt
+  - python manage.py test
+
+test-python37:
+  image: python:3.7
+  stage: test
+  script:
+  - pip install -r requirements.txt
+  - python manage.py test
+
 pages:
   image: centos:centos7
   stage: deploy