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. https://www.utahime.org/FreeBSD/USES_python_3_x.patch
I also created review. https://reviews.freebsd.org/D28666
Update patch to the diff to ports r565394. https://www.utahime.org/FreeBSD/USES_python_3_x.r565394.patch
Similar changes are made with ports 3d9a815d9c5a.