Codestyle fixes (flake8)

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

src/wagtail_personalisation/blocks.py

@@ -1,9 +1,10 @@
 from __future__ import absolute_import, unicode_literals
 
 from django.utils.translation import ugettext_lazy as _
-from wagtail_personalisation.models import Segment
 from wagtail.wagtailcore import blocks
 
+from wagtail_personalisation.models import Segment
+
 
 def list_segment_choices():
     for segment in Segment.objects.all():