Run flake8 on CI

Author: Hugo Osvaldo Barrera

.travis.yml

@@ -17,6 +17,11 @@ script:
 
 jobs:
   include:
+    - stage: test
+      before_script:
+        - pip install flake8 flake8-bugbear flake8-comprehensions flake8-import-order
+      script:
+        - flake8
     - stage: test
       python: 3.7
       dist: xenial