Override field propierties on inherited model in Django

model.py

from django.contrib.sites.models import Site as SiteOld

class Site(SiteOld):

    """Site only have a one Costumer/Shop."""
    costumer = models.OneToOneField('Costumer', verbose_name='customer')

    def __init__(self, *args, **kwargs):
        """
        Overrided __init__.
        Set unique constraints for domain and name fields
        Check in case of unique works with '_', 
        but blank and null don't
        """
        super(Site, self).__init__(self, *args, **kwargs)
        self._meta.get_field('domain')._unique = True
        self._meta.get_field('name')._unique = True
        self._meta.get_field('name').blank = False
        self._meta.get_field('name').null = False

    class Meta:
        verbose_name = _('site costumer')
        verbose_name_plural = _('sites costumers')