Merge branch 'gbp.conf' into 'pureos/byzantium'

d/gbp.conf: Update debian-tag for pureOS

See merge request Librem5/debs/pkg-chatty!3

debian/gbp.conf

 [DEFAULT]
 debian-branch = pureos/byzantium
+debian-tag = pureos/%(version)s
 upstream-branch = upstream/latest
 pristine-tar = true