dpkg-reconfigure cloud-init: yaml.load errors during MAAS deloyment of Ubuntu 23.04(Lunar)

Affects cloud-init 23.1.1 Lunar

MAAS deployed Ubuntu 23.04 (Lunar) machines invoke `dpkg-reconfigure cloud-init` which results in an exit 1 with the following traceback as seen in curtin logs:

Running command ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmpwr8y_4f8/target', 'dpkg-reconfigure', '--frontend=noninteractive', 'cloud-init'] with allowed return codes [0] (capture=True)
finish: cmd-install/stage-curthooks/builtin/cmd-curthooks/writing-apt-config: FAIL: configuring apt configuring apt
finish: cmd-install/stage-curthooks/builtin/cmd-curthooks: FAIL: curtin command curthooks
Traceback (most recent call last):
  File "/curtin/curtin/commands/main.py", line 202, in main
    ret = args.func(args)
          ^^^^^^^^^^^^^^^
  File "/curtin/curtin/commands/curthooks.py", line 1886, in curthooks
    builtin_curthooks(cfg, target, state)
  File "/curtin/curtin/commands/curthooks.py", line 1692, in builtin_curthooks
    do_apt_config(cfg, target)
  File "/curtin/curtin/commands/curthooks.py", line 97, in do_apt_config
    apt_config.handle_apt(apt_cfg, target)
  File "/curtin/curtin/commands/apt_config.py", line 73, in handle_apt
    apply_debconf_selections(cfg, target)
  File "/curtin/curtin/commands/apt_config.py", line 167, in apply_debconf_selections
    dpkg_reconfigure(need_reconfig, target=target)
  File "/curtin/curtin/commands/apt_config.py", line 133, in dpkg_reconfigure
    util.subp(['dpkg-reconfigure', '--frontend=noninteractive'] +
  File "/curtin/curtin/util.py", line 275, in subp
    return _subp(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/curtin/curtin/util.py", line 139, in _subp
    raise ProcessExecutionError(stdout=out, stderr=err,
curtin.util.ProcessExecutionError: Unexpected error while running command.
Command: ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmpwr8y_4f8/target', 'dpkg-reconfigure', '--frontend=noninteractive', 'cloud-init']
Exit code: 1
Reason: -
Stdout: ''
Stderr: Traceback (most recent call last):
          File "<string>", line 23, in <module>
        TypeError: load() missing 1 required positional argument: 'Loader'

Ubuntu 23.04 Lunar has published PyYAML 6.0.1[1] which finally changed the signature on yaml.load to require a Loader= parameter.

This traceback is due to a stale call to yaml.load() in cloud-init.postinst[2] that should have migrated to yaml.safe_load.

In Jammy and and earlier, PyYAML was still emitting deprecation warning messages to stderr, but those warnings are silenced automatically by any invocation to `dpkg-reconfigure --frontend=noninteractive cloud-init`. So none of the warnings showed up in curtin or MAAS, until we got an explicit non-zero exit code.

References:
[1] Pyyaml 6.0.1 commit sync'd to lunar https://git.launchpad.net/ubuntu/+source/pyyaml/commit/?id=5308dbefbe5dc7ce4a19adbfdb4c4e08a798217d
[2] yaml.load failure on Lunar: https://github.com/canonical/cloud-init/blob/ubuntu/devel/debian/cloud-init.postinst#L41