Codestyle fixes (flake8)

This also adds the flake8 dependencies to the test extras for
installation.

src/wagtail_personalisation/admin_urls.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 from django.conf.urls import url
+
 from wagtail_personalisation import views
 
 app_name = 'segment'