See #297: added black formatting check in CI

.gitlab-ci.yml

@@ -6,10 +6,20 @@ variables:
 
 
 stages:
+  - lint
   - test
   - build
   - deploy
 
+black:
+  image: python:3.6
+  stage: lint
+  before_script:
+    - pip install black
+  script:
+    - black --check api/
+
+
 test_api:
   services:
     - postgres:9.4