CI: add flake8

.gitlab-ci.yml

@@ -15,7 +15,10 @@ check_style:
   image: python:3.7-buster
   before_script:
     - pip install black
+    - pip install flake8
   script:
+    # run flake8
+    - flake8 nxtomomill --ignore=E501,W503,E203,E731
     # run black
     - LC_ALL=C.UTF-8 black --check --safe .