Fixed #21883 -- Added six.moves.xrange import

When moving code around from sql.where to lookups six.moves.xrange
import was forgotten. The xrange import is needed on Python 3.

django/db/models/lookups.py

@@ -5,6 +5,7 @@ import inspect
 from django.conf import settings
 from django.utils import timezone
 from django.utils.functional import cached_property
+from django.utils.six.moves import xrange
 
 
 class RegisterLookupMixin(object):