Fixed #20094 - Be more careful when checking for Iterator

Python 2.6 has some different behaviour when checking
isinstance(foo, collections.Iterator).

tests/utils_tests/models.py

@@ -1 +1,13 @@
-# Test runner needs a models.py file.
+from django.db import models
+
+
+class Category(models.Model):
+    name = models.CharField(max_length=100)
+
+    def next(self):
+        return self
+
+
+class Thing(models.Model):
+    name = models.CharField(max_length=100)
+    category = models.ForeignKey(Category)