CI: set python version to 3.5

.gitlab-ci.yml

@@ -1,3 +1,5 @@
+image: "python:3.5"
+
 services:
     - postgres:latest
     - redis:latest
@@ -34,6 +36,7 @@ before_script:
     # Remove the old test database if it has not been done yet
     - psql --username=$POSTGRES_USER --host=$DBHOST -c "DROP DATABASE IF EXISTS test_$POSTGRES_DB"
     - pip install --upgrade --cache-dir vendor/pip -t vendor/python -r requirements.txt
+    - python --version
 
 test:
     stage: test