Fixed #18269 -- Applied unicode_literals for Python 3 compatibility.

Thanks Vinay Sajip for the support of his django3 branch and
Jannis Leidel for the review.

tests/regressiontests/model_forms_regress/models.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import os
 
 from django.core.exceptions import ValidationError
@@ -13,7 +15,7 @@ class Triple(models.Model):
     right = models.IntegerField()
 
     class Meta:
-        unique_together = (('left', 'middle'), (u'middle', u'right'))
+        unique_together = (('left', 'middle'), ('middle', 'right'))
 
 class FilePathModel(models.Model):
     path = models.FilePathField(path=os.path.dirname(__file__), match=".*\.py$", blank=True)