debian: add dep on python modules

ade uses the argcomplete and xdg python modules.

Also add them as build-dep as ade is run during the build for testing.

Signed-off-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5214