3314fefaec started needing `python3-yaml`, but incorrectly claimed
that it was always an indirect dependency; it is a dependency of
`ubuntu-minimal` on 20.04, but not required on 18.04 or Debian. We
cannot install it in puppet because then is definitionally too late;
it is needed at load time by `zulip-puppet-apply`.
Install `python3-yaml`, but guarded by a simple check so as to not
further slow most installs.
Fixes#18179.
de41a10d38