flake8 fixes

wagtail/tests/testapp/models.py

@@ -533,6 +533,7 @@ class StreamPage(Page):
         StreamFieldPanel('body'),
     ]
 
+
 class MTIBasePage(Page):
     is_creatable = False
 

wagtail/wagtailcore/tests/test_tests.py

@@ -1,7 +1,5 @@
 from __future__ import absolute_import, unicode_literals
 
-import json
-
 from django.utils import six
 
 from wagtail.tests.testapp.models import (