Fixed #17627 -- Renamed util.py files to utils.py

Thanks PaulM for the suggestion and Luke Granger-Brown and
Wiktor Kołodziej for the initial patch.

django/db/models/options.py

@@ -77,7 +77,7 @@ class Options(object):
 
     def contribute_to_class(self, cls, name):
         from django.db import connection
-        from django.db.backends.util import truncate_name
+        from django.db.backends.utils import truncate_name
 
         cls._meta = self
         self.model = cls