d/gbp.conf: Adjust for byzantium

debian/gbp.conf

 [DEFAULT]
 pristine-tar = True
-debian-branch = debian/master
+debian-branch = pureos/byzantium
 upstream-branch = upstream/3.38.x
+debian-tag = pureos/%(version)s
+debian-tag-msg = %(pkg)s %(version)s
 
 [buildpackage]
 sign-tags = True
@@ -15,3 +17,6 @@ upstream-vcs-tag = GNOME_SETTINGS_DAEMON_%(version%.%_)s
 
 [pq]
 patch-numbers = False
+
+[tag]
+sign-tags = True