packaging: add newly-required python3 build dependency for client-qt

https://review.jami.net/c/jami-client-qt/+/19283 recently marked
Python 3 as a required build depdendency for building client-qt,
as it's needeed for generating 'resources.qrc' in build time.

Change-Id: Ib7eb3822ec5af5e4223d8a05b52d7cb5bb22e8d5