Fixed #32983 -- Added system check for redundant related_name on symmetrical M2M fields.

Since ManyToManyFields defined with `symmetrical=True` do not add a
related field to the target model, including a `related_name` argument
will never do what the coder likely expects. This makes including
a related_name with a symmetrical model raise a system check warning.

ticket-32983

tests/invalid_models_tests/test_relative_fields.py

@@ -128,6 +128,20 @@ class RelativeFieldTests(SimpleTestCase):
             ),
         ])
 
+    def test_many_to_many_with_useless_related_name(self):
+        class ModelM2M(models.Model):
+            m2m = models.ManyToManyField('self', related_name='children')
+
+        field = ModelM2M._meta.get_field('m2m')
+        self.assertEqual(ModelM2M.check(), [
+            DjangoWarning(
+                'related_name has no effect on ManyToManyField with '
+                'a symmetrical relationship, e.g. to "self".',
+                obj=field,
+                id='fields.W345',
+            ),
+        ])
+
     def test_ambiguous_relationship_model_from(self):
         class Person(models.Model):
             pass