Fixed flake8 warnings.

tests/test_utils/models.py

 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
+
 @python_2_unicode_compatible
 class Car(models.Model):
     name = models.CharField(max_length=100)
@@ -8,6 +9,7 @@ class Car(models.Model):
     def __str__(self):
         return self.name
 
+
 @python_2_unicode_compatible
 class Person(models.Model):
     name = models.CharField(max_length=100)
@@ -16,6 +18,7 @@ class Person(models.Model):
     def __str__(self):
         return self.name
 
+
 @python_2_unicode_compatible
 class PossessedCar(models.Model):
     car = models.ForeignKey(Car)