Travis CI: Add flake8 to the testing

.travis.yml

@@ -5,6 +5,13 @@ python:
 
 install:
   - "pip install -r requirements.txt -r dev_requirements.txt --timeout 45"
+  - 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:
   - "pytest tests/"