[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

django/db/models/fields/__init__.py

@@ -1272,9 +1272,13 @@ class DateTimeField(DateField):
             # For backwards compatibility, interpret naive datetimes in local
             # time. This won't work during DST change, but we can't do much
             # about it, so we let the exceptions percolate up the call stack.
-            warnings.warn("DateTimeField %s.%s received a naive datetime (%s)"
+            try:
+                name = '%s.%s' % (self.model.__name__, self.name)
+            except AttributeError:
+                name = '(unbound)'
+            warnings.warn("DateTimeField %s received a naive datetime (%s)"
                           " while time zone support is active." %
-                          (self.model.__name__, self.name, value),
+                          (name, value),
                           RuntimeWarning)
             default_timezone = timezone.get_default_timezone()
             value = timezone.make_aware(value, default_timezone)