Enforce flake8 in CI.

.circleci/config.yml

@@ -96,9 +96,10 @@ jobs:
             - checkout
             - run: sudo pip install --editable .
             - run: sudo pip install torch tensorflow tensorboardX scikit-learn
-            - run: sudo pip install black git+git://github.com/timothycrosley/isort.git@e63ae06ec7d70b06df9e528357650281a3d3ec22#egg=isort
+            - run: sudo pip install black git+git://github.com/timothycrosley/isort.git@e63ae06ec7d70b06df9e528357650281a3d3ec22#egg=isort flake8
             - run: black --check --line-length 119 examples templates transformers utils
             - run: isort --check-only --recursive examples templates transformers utils
+            - run: flake8 examples templates transformers utils
     check_repository_consistency:
         working_directory: ~/transformers
         docker: