Revert "debian: dh no longer knows "--with python3""

This reverts commit 2ca0603f.
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

debian/rules

@@ -7,7 +7,7 @@
 export DEB_BUILD_MAINT_OPTIONS=hardening=+all
 
 %:
-	dh $@ --with-systemd
+	dh $@ --with python3 --with-systemd
 
 override_dh_auto_configure:
 	intltoolize --force --automake