Update flake8 from 3.2.1 to 3.3.0

Author: pyup-bot

requirements-dev.txt

@@ -7,7 +7,7 @@
 -e .
 coverage==4.3.4
 django-appconf==1.0.2
-flake8==3.2.1
+flake8==3.3.0
 isort==4.2.5
 mccabe==0.6.1 # via flake8
 pydocstyle==1.1.1