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/expressions/tests.py

@@ -3,6 +3,7 @@ import pickle
 import unittest
 import uuid
 from copy import deepcopy
+from unittest import mock
 
 from django.core.exceptions import FieldError
 from django.db import DatabaseError, connection, models
@@ -965,6 +966,7 @@ class SimpleExpressionTests(SimpleTestCase):
             Expression(models.IntegerField()),
             Expression(output_field=models.IntegerField())
         )
+        self.assertEqual(Expression(models.IntegerField()), mock.ANY)
         self.assertNotEqual(
             Expression(models.IntegerField()),
             Expression(models.CharField())