[CI] add format stage in .gitlab-ci.yml

.gitlab-ci.yml

 stages:
+  - format
   - tests
   - build
   - release
@@ -52,6 +53,16 @@ image: registry.duniter.org/docker/python3/duniterpy-builder:0.0.7
       - tests/**/*.py
 
 # TASKS
+format:
+  extends:
+    - .env
+    - .changes
+  stage: format
+  script:
+    - pyenv shell 3.6.4  # black install and run needs python 3.6.x minimum
+    - pip install -r requirements_dev.txt
+    - make check-format
+
 tests:
   extends:
     - .env