Can't upgrade charm to latest/edge (674712d) on Focal

[James Simpson]

I'm seeing the following issue when trying to upgrade this charm to latest/edge (rev 23, version 674712d) on a Focal box:

2022-12-12 03:30:10 WARNING unit.alertmanager/0.upgrade-charm logger.go:60 ImportError: cannot import name 'soft_unicode' from 'markupsafe' (/var/lib/juju/agents/unit-alertmanager-0/.venv/lib/python3.8/site-packages/markupsafe/__init__.py)

From a bit of reading (https://github.com/aws/aws-sam-cli/issues/3661) it sounds like this issue is introduced in version 2.1.0

Currently we're installing version 2.1.1:

2022-12-12 03:36:51 DEBUG unit.prometheus-alertmanager/0.install logger.go:60 Successfully installed Jinja2-2.11.3 MarkupSafe-2.1.1 PyYAML-5.3.1 charmhelpers-1.2.1 charms.reactive-1.5.1 flit-core-3.8.0 netaddr-0.7.19 packaging-21.3 pbr-5.11.0 pyaml-21.10.1 pyparsing-3.0.9 six-1.16.0 tenacity-5.0.3 tomli-2.0.1

It looks likely that until this upstream issue[1] in layer-basic is resolved, similar changes to [2] will be required here.

[1] https://github.com/juju-solutions/layer-basic/issues/210
[2] https://github.com/juju-solutions/layer-basic/pull/211/files