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.
on whatever is the current default version of python 3 in the ports tree
(currently 3.7). Allows adding it with pkg_add python3 rather than needing to
know the correct subdir name (as you would with pkg_add python%3.7).
Deliberately not adding a similar one for py2 which is not likely to
proceed beyond 2.7.
ok kmos@, aja@ pirofti@ zhuk@ like it too
