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.
as discussed with robert@ and sthen@, chromium is big, and electron
is mostly an independent port, so tieing up dpb twice is no fun,
even if electron is slightly smaller.
this is mostly a simplified carbon-copy of the www/chromium,electron port.
I still want to give it another look before switching to that one.
