Add TinyIntegerField and PositiveTinyIntegerField

[UnknownPlatypus]

Description

Hi Adam,

This is a small feature request about adding support for the mysql type TYNYINT with 2 fields TinyIntegerField and PositiveTinyIntegerField. I would be happy to provide a PR for this.

I found a related discussion in #749, however I am unsure this belongs as a Django core feature for the TINYINT case because postgresql (which I believe is the most used django DB backend) doesn't have a TINYINT type, only the 2 bytes SMALLINT type.

Thanks in advance

[adamchainz]

Sure, we can do that. The existing fields should hopefully give you enough pointers as to how to write the code, docs, and tests! Good luck.