xarray: Dataset.drop() no longer works with a set as an argument

MCVE Code Sample

# Your code here
#!/usr/bin/env python

import xarray
import numpy

ds = xarray.Dataset()
ds['x'] = ('x', numpy.linspace(0., 1., 100))
ds['y'] = ('y', numpy.linspace(0., 1., 50))
print(ds)
ds = ds.drop(set('x'))
print(ds)

Expected Output

<xarray.Dataset>
Dimensions:  (x: 100, y: 50)
Coordinates:
  * x        (x) float64 0.0 0.0101 0.0202 0.0303 ... 0.9697 0.9798 0.9899 1.0
  * y        (y) float64 0.0 0.02041 0.04082 0.06122 ... 0.9592 0.9796 1.0
Data variables:
    *empty*
<xarray.Dataset>
Dimensions:  (y: 50)
Coordinates:
  * y        (y) float64 0.0 0.02041 0.04082 0.06122 ... 0.9592 0.9796 1.0
Data variables:
    *empty*

Problem Description

In versions before xarray 0.14.1, the code above involving calls to Dataset.drop() with a set as an argument worked as expected. With the new release, the result is an error as shown below. This is breaking backwards compatibility with our software (MPAS-Analysis).

<xarray.Dataset>
Dimensions:  (x: 100, y: 50)
Coordinates:
  * x        (x) float64 0.0 0.0101 0.0202 0.0303 ... 0.9697 0.9798 0.9899 1.0
  * y        (y) float64 0.0 0.02041 0.04082 0.06122 ... 0.9592 0.9796 1.0
Data variables:
    *empty*
Traceback (most recent call last):
  File "./drop_issue.py", line 10, in <module>
    ds = ds.drop(set('x'))
  File "/home/xylar/miniconda3/envs/test/lib/python3.7/site-packages/xarray/core/dataset.py", line 3643, in drop
    return self.drop_sel(labels, errors=errors)
  File "/home/xylar/miniconda3/envs/test/lib/python3.7/site-packages/xarray/core/dataset.py", line 3689, in drop_sel
    labels = either_dict_or_kwargs(labels, labels_kwargs, "drop")
  File "/home/xylar/miniconda3/envs/test/lib/python3.7/site-packages/xarray/core/utils.py", line 257, in either_dict_or_kwargs
    "the first argument to .%s must be a dictionary" % func_name
ValueError: the first argument to .drop must be a dictionary

Output of xr.show_versions()

# Paste the output here xr.show_versions() here INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 21:52:21) [GCC 7.3.0] python-bits: 64 OS: Linux OS-release: 4.15.0-1063-oem machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.7.1

xarray: 0.14.1 pandas: 0.25.3 numpy: 1.17.3 scipy: 1.3.2 netCDF4: 1.5.3 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.0.4.2 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: 1.3.1 dask: 2.8.0 distributed: 2.8.0 matplotlib: 3.1.2 cartopy: 0.17.0 seaborn: None numbagg: None setuptools: 41.6.0.post20191101 pip: 19.3.1 conda: None pytest: 5.3.0 IPython: None sphinx: None