Fixed #29979, Refs #17337 -- Extracted AutoField field logic into a mixin and refactored AutoFields.

This reduces duplication by allowing AutoField, BigAutoField and
SmallAutoField to inherit from IntegerField, BigIntegerField and
SmallIntegerField respectively. Doing so also allows for enabling the
max_length warning check and minimum/maximum value validation for auto
fields, as well as providing a mixin that can be used for other possible
future auto field types such as a theoretical UUIDAutoField.

tests/invalid_models_tests/test_ordinary_fields.py

@@ -38,6 +38,21 @@ class AutoFieldTests(SimpleTestCase):
             ),
         ])
 
+    def test_max_length_warning(self):
+        class Model(models.Model):
+            auto = models.AutoField(primary_key=True, max_length=2)
+
+        field = Model._meta.get_field('auto')
+        self.assertEqual(field.check(), [
+            DjangoWarning(
+                "'max_length' is ignored when used with %s."
+                % field.__class__.__name__,
+                hint="Remove 'max_length' from field",
+                obj=field,
+                id='fields.W122',
+            ),
+        ])
+
 
 @isolate_apps('invalid_models_tests')
 class BinaryFieldTests(SimpleTestCase):