related name in Django abstract model using python 3

Question

When I use %(app_label)s_%(class)s in related name using Python3.4 I got the following error

    'app_label': cls._meta.app_label.lower()
TypeError: unsupported operand type(s) for %: 'bytes' and 'dict'

is there another way for that ?

edit

class TimeStampModel(models.Model):     

    modified = models.DateTimeField(auto_now=True)
    created = models.DateTimeField(auto_now_add=True)

    created_by = models.ForeignKey(User, related_name = "%(app_label)s_%(class)s_created_by" ,)
    edited_by =  models.ForeignKey(User, related_name = "%(app_label)s_%(class)s_edited_by" ,  blank=True, null=True)

    class Meta:
        abstract = True