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
1171069302,PR_kwDOAMm_X840iQxK,6368,"#6367 Fix for time units checking could produce ""unhashable type"" error",212077,closed,0,,,2,2022-03-16T14:02:56Z,2022-03-18T21:21:20Z,2022-03-18T21:18:52Z,CONTRIBUTOR,,0,pydata/xarray/pulls/6368,"<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Closes #6367
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1170989744,I_kwDOAMm_X85Fy-Kw,6367,unhashable type: 'numpy.ndarray' if 'units' attribute is numpy array in NetCDF file,212077,closed,0,,,0,2022-03-16T12:59:25Z,2022-03-18T21:18:51Z,2022-03-18T21:18:51Z,CONTRIBUTOR,,,,"### What happened?

Calling `load_dataset()` for a file with variable with the 'units' attribute being numpy.ndarray as following:

```py
{'paramId': array([3015], dtype=int64), 
'typeOfLevel': 'heightAboveGround', 
'gridType': 'regular_ll', 
'bottomLevel': array([2], dtype=int64), 
'cfVarName': 'tmax', 
'missingValue': array([9999], dtype=int64), 
'name': 'Maximum temperature', 
'scaledValueOfFirstFixedSurface': array([2], dtype=int64), 
'scaledValueOfSecondFixedSurface': array([0], dtype=int64), 
'shortName': 'tmax', 
'topLevel': array([2], dtype=int64), 
'units': array(['˚C'], dtype=object), 
'max': array(['47.22 ˚C'], dtype=object), 
'min': array(['-59.20 ˚C'], dtype=object), 
'comment': 'Daily max'}
```

Causes error ""unhashable type: 'numpy.ndarray'"" https://github.com/pydata/xarray/blob/95bb9ae4233c16639682a532c14b26a3ea2728f3/xarray/coding/times.py#L698, because attrs['units'] is a numpy.ndarray.



### What did you expect to happen?

I guess another check could be added to ignore the attribute if its value is unhashable

### Minimal Complete Verifiable Example

_No response_

### Relevant log output

_No response_

### Anything else we need to know?

_No response_

### Environment

xr.show_versions()
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3 | packaged by conda-forge | (default, Jul  1 2019, 22:01:29) [MSC v.1900 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: (None, None)
libhdf5: 1.10.6
libnetcdf: 4.7.4
xarray: 0.19.0
pandas: 1.3.2
numpy: 1.21.5
scipy: 1.6.0
netCDF4: 1.5.6
pydap: None
h5netcdf: 0.11.0
h5py: 3.2.1
Nio: None
zarr: None
cftime: 1.5.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.7
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.5.1
cartopy: 0.18.0
seaborn: None
numbagg: None
pint: None
setuptools: 59.8.0
pip: 21.0.1
conda: None
pytest: 6.2.5
IPython: None
sphinx: 4.1.0
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