Add python3-systemd package

Otherwise journal logging under python3 doesn't work

Change-Id: Ib136d88a522c40482a3e94d0386a26600236f135
diff --git a/files/debs/general b/files/debs/general
index 3a0e241..edf64ce 100644
--- a/files/debs/general
+++ b/files/debs/general
@@ -23,6 +23,7 @@
 pkg-config
 psmisc
 python2.7
+python3-systemd
 python-dev
 python-gdbm # needed for testr
 python-systemd