`humanize_list` output is incorrect when list contains only one item

### Bug Description

When `humanize_list` is run with only a single item, it returns an invalid value.

```
>>> craft_cli.utils.humanize_list(['1'])
', and 1'
```

### To Reproduce

```
craft_cli.utils.humanize_list(['1'])
```

### part yaml

```shell

```

### Relevant log output

```shell
n/a
```