Install OpenPGP Applet from jessie-backports, instead of from Stretch.
Currently our devel branch FTBFS since Stretch got openpgp-applet 1.0,
that has a new dependency. For now let's revert to 0.9.1, that is the
version we installed before openpgp-applet 1.0 migrated to testing,
modulo "~bpo8+1". Ticket #11899 is there to track the upgrade to 1.0.