Travis CI: Flake8 spots some missing imports

.travis.yml

@@ -71,6 +71,12 @@ before_install:
     fi
 install:
   - bash ./scripts/ci/install.sh
+  - pip install flake8
+before_script:
+  # stop the build if there are Python syntax errors or undefined names
+  - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+  # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+  - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
 script:
   - bash ./scripts/ci/script.sh
 deploy: