matrix: Give pantalaimon a few seconds to establish its listeners

To prevent mjolnir from failing.

roles/matrix/files/pantalaimon.service

@@ -9,6 +9,7 @@ WorkingDirectory=~
 ExecStart=/var/lib/synapse/venv-pantalaimon/bin/pantalaimon \
   -c /etc/synapse/pantalaimon.conf \
   --data-path /var/lib/synapse/pantalaimon-data
+ExecStartPost=/usr/bin/sleep 3
 
 [Install]
 WantedBy=default.target