`humanize_list` does not properly handle two-item lists.

### Bug Description

Because `humanize_list` insists on a serial comma, a two-item list is incorrectly converted.

### To Reproduce

```
>>> craft_cli.utils.humanize_list(["a", "b"])
'a, and b'
```

### part yaml

```shell

```

### Relevant log output

```shell
n/a
```