id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
787847449,MDU6SXNzdWU3ODc4NDc0NDk=,4821,Error when supplying a tuple of dimensions to DataArray.sortby(),5158900,open,0,,,18,2021-01-18T01:22:08Z,2021-06-13T17:28:13Z,,NONE,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**What happened**:

A KeyError is thrown when supplying a tuple of dimensions to DataArray.sortby()

**What you expected to happen**:

The dataarray to be sorted according to the dimensions.

**Minimal Complete Verifiable Example**:

```python
import xarray as xr
import numpy as np

da=xr.DataArray(np.random.rand(3,3), coords=(('x', range(3, 0, -1)), ('y', range(3, 0, -1))))
da.sortby(da.dims)
```

**Anything else we need to know?**:

If the tuple is cast to a list it works correctly:

```python
da.sortby(list(da.dims))
```


**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

<!-- Paste the output here xr.show_versions() here -->

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.12 (default, Nov 11 2020, 22:22:08) 
[GCC 4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.46.4)]
python-bits: 64
OS: Darwin
OS-release: 18.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None
LOCALE: en_US.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.16.2
pandas: 1.1.4
numpy: 1.19.4
scipy: 1.5.4
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2020.12.0
distributed: None
matplotlib: 3.3.3
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 50.3.2
pip: 20.3.3
conda: None
pytest: 6.2.1
IPython: 7.16.1
sphinx: None

</details>
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