Fix desktuils/py-khal after upgrade of devel/py-dateutil

The upgrade of devel/py-dateutil in r475926 to 2.7.3 broke packaging of
desktuils/py-khal as that one pinned to <= 2.6.1.

• remove the pinning of the version (0.9.9 should already be fixed to work with 2.7.x -- even if the information on that seems to be a bit conflicting)

• while here, do some cosmetics on pkg-message

Submitted by: lebarondemerde@privacychain.ch (maintainer)