Coordinate attrs dropped when using sel with units

Coordinate attrs are not kept when using pint.sel.

```
import xarray as xr
from pint_xarray import unit_registry

ds = xr.Dataset(
    {
        "y": ("y", [60, 120], {"units": unit_registry.Unit("s"), "long_name": "time"}),
    }
)

ds.pint.quantify().pint.sel(y=120 * unit_registry.Unit("s")).y.long_name
```

raises an AttributeError.

I am submitting a pull request to fix.