This should be fixed now in pkgsrc -current, as python27-2.7.18nb5.
John, if you're able to test it resolves your issue, please let me know
your findings. (I don't see why it wouldn't work now with nb5, but I
don't use mailman.) Sorry about this mistake.
Technically, the issue was that _QS_SEPARATOR_CONFIG_FILENAME wasn't
defined at all, as I'd noticed the hard-coded '/etc', and had commented
it out, intending to adjust it further later. But I forgot to actually
do so before I committed the full change set.