if a port needs 2.x then set MODPY_VERSION=${MODPY_DEFAULT_VERSION_2}.
This commit doesn't change any versions currently used; it may be that
some ports have MODPY_DEFAULT_VERSION_2 but don't require it, those
should be cleaned up in the course of updating ports where possible.
Python module ports providing py3-* packages should still use
FLAVOR=python3 so that we don't have a mixture of dependencies some
using ${MODPY_FLAVOR} and others not.
Follow the upstream recommendations for packagers and switch to
multi-packages:
devel/gettext -> devel/gettext,-runtime
devel/gettext-tools -> devel/gettext,-tools
(new) devel/gettext,-textstyle
