[1.7.x] Fixed flake8 warnings.

Backport of 9a922dcad1 from master

django/db/migrations/autodetector.py

@@ -716,7 +716,6 @@ class MigrationAutodetector(object):
                             self.renamed_fields[app_label, model_name, field_name] = rem_field_name
                             break
 
-
     def generate_added_fields(self):
         """
         Fields that have been added

tests/model_fields/models.py

@@ -58,7 +58,7 @@ class Counter(six.Iterator):
             raise StopIteration
         else:
             self.n += 1
-            return (self.n, 'val-'+str(self.n))
+            return (self.n, 'val-' + str(self.n))
 
 
 class WhizIter(models.Model):