textproc/py-os-api-ref: Update to 3.1.0

• Adjust USES=python accordingly, because Python 3.8 is no longer present in the ports tree after ca15850a2b13 while I'm here.

• Switch to the PEP517 build framework as well.

Changelog:

https://github.com/openstack/os-api-ref/compare/3.0.1...3.1.0