What happened?

When using DataArray/Dataset.isel with drop=True with a scalar DataArray as indexer (see example) resulting scalar coordinates do not get dropped. When using an integer the behavior is as expected.

What did you expect to happen?

I expect that using a scalar DataArray behaves the same as an integer.

Minimal Complete Verifiable Example

```Python import xarray as xr

da = xr.DataArray([1, 2, 3], dims="x", coord={"k": ("x", [0, 1, 2])})

<xarray.DataArray (x: 3)>

array([1, 2, 3])

Coordinates:

k (x) int32 0 1 2

da.isel({"x": 1}, drop=True)

works

<xarray.DataArray ()>

array(2)

da.isel({"x": xr.DataArray(1)}, drop=True)

does not drop "k" coordinate

<xarray.DataArray ()>

array(2)

Coordinates:

k int32 1

```

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: 4fbca23a9fd8458ec8f917dd0e54656925503e90 python: 3.9.6 | packaged by conda-forge | (default, Jul 6 2021, 08:46:02) [MSC v.1916 64 bit (AMD64)] python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: AMD64 Family 23 Model 113 Stepping 0, AuthenticAMD byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('de_DE', 'cp1252') libhdf5: 1.10.6 libnetcdf: 4.7.4 xarray: 0.18.2.dev76+g3a7e7ca2.d20210706 pandas: 1.3.0 numpy: 1.21.0 scipy: 1.7.0 netCDF4: 1.5.6 pydap: installed h5netcdf: 0.11.0 h5py: 3.3.0 Nio: None zarr: 2.8.3 cftime: 1.5.0 nc_time_axis: 1.3.1 PseudoNetCDF: installed cfgrib: None iris: 2.4.0 bottleneck: 1.3.2 dask: 2021.06.2 distributed: 2021.06.2 matplotlib: 3.4.2 cartopy: 0.19.0.post1 seaborn: 0.11.1 numbagg: 0.2.1 fsspec: 2021.06.1 cupy: None pint: 0.17 sparse: 0.12.0 setuptools: 49.6.0.post20210108 pip: 21.3.1 conda: None pytest: 6.2.4 IPython: None sphinx: None