After the commit of ports r564032, 'USES=python' without version-spec means 'USES=python:3.6+'. So remove unnecessary '3.[0-6]+' argument of 'USES=python' from Makefiles.
Oops, patch is to big and exceeds the limit of max size. So I put it to following URL.
I also created review.
Update patch to the diff to ports r565394.