[1.7.x] Fixed #23420 - broken warning for unbound naive datetime objects

Fixed issue with warning message displayed for unbound naive datetime
objects when USE_TZ is True. Adds unit test that demonstrates the issue
(discoverable when using a custom lookup in MySQL).

Backport of ceb1ffcc8d from master.

Conflicts:
	tests/custom_lookups/tests.py

tests/custom_lookups/models.py

@@ -11,3 +11,11 @@ class Author(models.Model):
 
     def __str__(self):
         return self.name
+
+
+@python_2_unicode_compatible
+class MySQLUnixTimestamp(models.Model):
+    timestamp = models.PositiveIntegerField()
+
+    def __str__(self):
+        return self.name