Change Field to TextField

[Diegovsky]

While trying to use HashidsField with DRF and django-spectacular (DS) (an OpenAPI schema generator for DRF), I got errors which accused HashidsField of not mapping correctly into any specific OpenAPI type.

This stems from the fact that HashidsField, while for all intents and purposes acts like a str, does not have a way to tell this to DS. A simple fix would be to make HashidsField inherit from TextField instead of Field. I believe this is correct but I'm open to feedback.

I'm not familiar enough with the django ecosystem to understand how to run the tests from this repo, but at a first glance, basic queries worked flawlessly in my project.