Using systemd-python allows for logging to the systemd journal,
as is documented in: `synapse/contrib/systemd/log_config.yaml`.

Signed-off-by: Silke Hofstra <silke@slxh.eu>

Make sure it refreshes the apt cache before trying to install stuff

since #4298, the optional dependencies are no longer installed with a simple
`pip install .`, which meant that they were not being included in the debian
package.

The easy fix to that is dh_virtualenv --extras, but that needs dh_virtualenv
1.1...