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
65a6d83bcd