Update flake8 from 3.5.0 to 3.6.0

Author: pyup-bot

requirements_dev.txt

@@ -2,7 +2,7 @@ pip==18.0
 bumpversion==0.5.3
 wheel==0.31.1
 watchdog==0.9.0
-flake8==3.5.0
+flake8==3.6.0
 tox==3.4.0
 coverage==4.5.1
 Sphinx==1.8.0