related name in Django abstract model using python 3 -

when use %(app_label)s_%(class)s in related name using python3.4 got following error

    'app_label': cls._meta.app_label.lower() typeerror: unsupported operand type(s) %: 'bytes' , 'dict' 

is there way ?

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