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.
Newer versions of py-cairo dropped support for python2, sadly
x11/py-gtk2 still needs a python2 cairo module.
Create a python2-only port of the existing version of py-cairo under
a new path, to be used by x11/py-gtk2. Then ajacoutot@ can move
graphics/py-cairo to newer releases.
Suggested by sthen@, discussed with ajacoutot@
