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.
Adam Johnson
David-Wobrock
Nick Pope
Diederik van der Boor