Deleting Sites through a manager does not clear cache
When you delete a Site instance, i.e.
Site.objects.get_current().delete(), the cache is cleared. However, if you delete all sites,
Site.objects.all().delete(), the cache is not cleared.
When all site objects are deleted the cache should also
be cleared and get_current should raise a DoesNotExist
from django.contrib.sites.models import Site
site = Site.objects.create(domain='example.com', name='test')