0663b23d54 changed zulip-puppet-apply to
use the venv, because it began using `yaml` to parse the output of
puppet to determine if changes would happen.
However, not every install ends with a venv; notably, non-frontend
servers do not have one. Attempting to run zulip-puppet-apply on them
hence now fails.
Remove this dependency on the venv, by installing a system
python3-yaml package -- though in reality, this package is already an
indirect dependency of the system. Especially since pyyaml is quite
stable, we're not using it in any interesting way, and it does not
actually add to the dependencies, it is preferable to parsing the YAML
by hand in this instance.
3314fefaec