Fixed #30651 -- Made __eq__() methods return NotImplemented for not implemented comparisons.

Changed __eq__ to return NotImplemented instead of False if compared to
an object of the same type, as is recommended by the Python data model
reference. Now these models can be compared to ANY (or other objects
with __eq__ overwritten) without returning False automatically.

tests/model_indexes/tests.py

@@ -1,3 +1,5 @@
+from unittest import mock
+
 from django.conf import settings
 from django.db import connection, models
 from django.db.models.query_utils import Q
@@ -28,6 +30,7 @@ class SimpleIndexesTests(SimpleTestCase):
         same_index.model = Book
         another_index.model = Book
         self.assertEqual(index, same_index)
+        self.assertEqual(index, mock.ANY)
         self.assertNotEqual(index, another_index)
 
     def test_index_fields_type(self):