Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Create a project that uses a django 2.1.x PositiveIntegerField in the api. Use drf to generate an openapi spec

Expected behavior

the field should be represented as a type field of integer and there should be a format field of int64

Actual behavior

the field has type field of integer, but there is no format field of int64