[1.7.x] Fixed flake8 warning.

Backport of dec7da3c from master

tests/model_forms/tests.py

@@ -2351,6 +2351,7 @@ class CustomFieldWithQuerysetButNoLimitChoicesTo(forms.Field):
 
 class StumpJokeWithCustomFieldForm(forms.ModelForm):
     custom = CustomFieldWithQuerysetButNoLimitChoicesTo()
+
     class Meta:
         model = StumpJoke
         fields = ()  # We don't need any fields from the model