Enable warnings-as-errors across charmlibs

Ideally, all the packages would have warnings promoted to errors when running unit tests. Something like this in pyproject.toml:

```toml
[tool.pytest.ini_options]
filterwarnings = [
"error",
]
```

It would be useful to first see how much breakage this causes. If none/minimal, then my preference would be to do it across the whole repo. If not, then probably this becomes a package-by-package job.

I'd like to see this as the default for new packages, and, once it's adopted throughout the repo, maybe even enforced.