www/py-drf-yasg: Remove COREAPI option

• The COREAPI option depends on devel/py-coreapi, which is slated for removal since 19213c610945. Therefore, drop the COREAPI option in preparation for the removal of devel/py-coreapi which should happen in the next few days.

• Also switch to the PEP517 build framework and bump PORTREVISION due changed package contents.