Support label-based slicing in RangeIndex

### What happened?

```python
import xarray as xr

idx1 = xr.indexes.RangeIndex.arange(0.0, 1000.0, 1e-9, dim="x")
ds1 = xr.Dataset(coords=xr.Coordinates.from_xindex(idx1))
ds1.sel(x=slice(1, 2))
```

raises 
```
ValueError: RangeIndex only supports selection with method='nearest'
```

I think this should work