the duply ports depends on sysutils/duplicity ports. However duplicity is still a python 2.7 program. sysutils/duplicity-devel port is the upstream development version which uses python 3
It would be useful to be able to install duply with duplicity-devel instead of duplicity
devel/duplicity is now ahead of duplicity-devel and also supports Python 3, so this PR can be closed. Also, duplicity-devel expired today.