No longer test on python 3.5

Don't test Django 2.1 yet as that fails

.gitlab-ci.yml

@@ -8,8 +8,8 @@ variables:
   # Installs:
   #  - pip: tox, coverage, pipenv
   #  - apt: ghostscript
-  PY35_IMAGE: thalia/python-thalia:3.5
   PY36_IMAGE: thalia/python-thalia:3.6
+  PY37_IMAGE: thalia/python-thalia:3.7
   PIP_CACHE_DIR: "${CI_PROJECT_DIR}/pip-cache"
 
 stages:
@@ -18,7 +18,7 @@ stages:
 
 pep8:
   stage: test
-  image: $PY35_IMAGE
+  image: $PY36_IMAGE
   script:
     - tox -e flake8
 
@@ -33,23 +33,23 @@ pep8:
     - cd website
     - coverage report
 
-python35-django20:
+python36-django20:
   <<: *djangotest
-  image: $PY35_IMAGE
+  image: $PY36_IMAGE
   variables:
-    PYTHON_VERSION: py35
+    PYTHON_VERSION: py36
     DJANGO_VERSION: django20
 
-python36-django20:
+python37-django20:
   <<: *djangotest
-  image: $PY36_IMAGE
+  image: $PY37_IMAGE
   variables:
-    PYTHON_VERSION: py36
+    PYTHON_VERSION: py37
     DJANGO_VERSION: django20
 
 docs:
   stage: test
-  image: $PY35_IMAGE
+  image: $PY36_IMAGE
   before_script:
     # install django deps
     - pipenv --bare install --dev --ignore-pipfile