`DataGroup.__setitem__` fails to prevent non-string keys

```python
import scipp as sc

ds = sc.DataGroup(a=sc.arange('x', 4))
ds[1] = sc.scalar(666)  # User expects to set a slice
ds._items  # Item with int key was added
```
Note that things such as `DataGroup._repr_html_` raise a weird error with such a `DataGroup`.
