issues: 510326302

I just updated to the latest cftime release, and all of a sudden sel() appears to be broken:

```python In [1]: import xarray as xr

In [2]: import cftime

In [3]: ds = xr.tutorial.load_dataset('rasm')

In [4]: ds
Out[4]: <xarray.Dataset> Dimensions: (time: 36, x: 275, y: 205) Coordinates: * time (time) object 1980-09-16 12:00:00 ... 1983-08-17 00:00:00 xc (y, x) float64 189.2 189.4 189.6 189.7 ... 17.65 17.4 17.15 16.91 yc (y, x) float64 16.53 16.78 17.02 17.27 ... 28.26 28.01 27.76 27.51 Dimensions without coordinates: x, y Data variables: Tair (time, y, x) float64 nan nan nan nan nan ... 29.8 28.66 28.19 28.21 Attributes: title: /workspace/jhamman/processed/R1002RBRxaaa01a/l... institution: U.W. source: RACM R1002RBRxaaa01a output_frequency: daily output_mode: averaged convention: CF-1.4 references: Based on the initial model of Liang et al., 19... comment: Output from the Variable Infiltration Capacity... nco_openmp_thread_number: 1 NCO: "4.6.0" history: Tue Dec 27 14:15:22 2016: ncatted -a dimension...

In [5]: ds.sel(time=slice("1980", "1982"))

ValueError Traceback (most recent call last) <ipython-input-5-2c26e36a673a> in <module> ----> 1 ds.sel(time=slice("1980", "1982"))

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/dataset.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs) 1998 indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "sel") 1999 pos_indexers, new_indexes = remap_label_indexers( -> 2000 self, indexers=indexers, method=method, tolerance=tolerance 2001 ) 2002 result = self.isel(indexers=pos_indexers, drop=drop)

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/coordinates.py in remap_label_indexers(obj, indexers, method, tolerance, **indexers_kwargs) 390 391 pos_indexers, new_indexes = indexing.remap_label_indexers( --> 392 obj, v_indexers, method=method, tolerance=tolerance 393 ) 394 # attach indexer's coordinate to pos_indexers

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/indexing.py in remap_label_indexers(data_obj, indexers, method, tolerance) 259 coords_dtype = data_obj.coords[dim].dtype 260 label = maybe_cast_to_coords_dtype(label, coords_dtype) --> 261 idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance) 262 pos_indexers[dim] = idxr 263 if new_idx is not None:

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/core/indexing.py in convert_label_indexer(index, label, index_name, method, tolerance) 123 _sanitize_slice_element(label.start), 124 _sanitize_slice_element(label.stop), --> 125 _sanitize_slice_element(label.step), 126 ) 127 if not isinstance(indexer, slice):

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in slice_indexer(self, start, end, step, kind) 5032 slice(1, 3) 5033 """ -> 5034 start_slice, end_slice = self.slice_locs(start, end, step=step, kind=kind) 5035 5036 # return a slice

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in slice_locs(self, start, end, step, kind) 5246 start_slice = None 5247 if start is not None: -> 5248 start_slice = self.get_slice_bound(start, "left", kind) 5249 if start_slice is None: 5250 start_slice = 0

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in get_slice_bound(self, label, side, kind) 5158 # For datetime indices label may be a string that has to be converted 5159 # to datetime boundary according to its resolution. -> 5160 label = self._maybe_cast_slice_bound(label, side, kind) 5161 5162 # we need to look up the label

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _maybe_cast_slice_bound(self, label, side, kind) 336 pandas.tseries.index.DatetimeIndex._maybe_cast_slice_bound""" 337 if isinstance(label, str): --> 338 parsed, resolution = _parse_iso8601_with_reso(self.date_type, label) 339 start, end = _parsed_string_to_bounds(self.date_type, resolution, parsed) 340 if self.is_monotonic_decreasing and len(self) > 1:

~/opt/miniconda3/envs/intake-esm-dev/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _parse_iso8601_with_reso(date_type, timestr) 114 # 1.0.3.4. 115 replace["dayofwk"] = -1 --> 116 return default.replace(**replace), resolution 117 118

cftime/_cftime.pyx in cftime._cftime.datetime.replace()

ValueError: Replacing the dayofyr or dayofwk of a datetime is not supported.

```

Output of xr.show_versions()

# Paste the output here xr.show_versions() here ```python In [6]: xr.show_versions() INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 14:38:56) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.7.1 xarray: 0.14.0 pandas: 0.25.2 numpy: 1.17.2 scipy: None netCDF4: 1.5.1.2 pydap: None h5netcdf: None h5py: None Nio: None zarr: 2.3.2 cftime: 1.0.4 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.2.0 distributed: 2.5.1 matplotlib: None cartopy: None seaborn: None numbagg: None setuptools: 41.4.0 pip: 19.2.1 conda: None pytest: 5.0.1 IPython: 7.8.0 sphinx: 2.1.2 ```

Expected Output

I can confirm that everything works just fine with an older version of cftime:

```python In [4]: ds.sel(time=slice("1980", "1982"))
Out[4]: <xarray.Dataset> Dimensions: (time: 28, x: 275, y: 205) Coordinates: * time (time) object 1980-09-16 12:00:00 ... 1982-12-17 00:00:00 xc (y, x) float64 ... yc (y, x) float64 ... Dimensions without coordinates: x, y Data variables: Tair (time, y, x) float64 ... Attributes: title: /workspace/jhamman/processed/R1002RBRxaaa01a/l... institution: U.W. source: RACM R1002RBRxaaa01a output_frequency: daily output_mode: averaged convention: CF-1.4 references: Based on the initial model of Liang et al., 19... comment: Output from the Variable Infiltration Capacity... nco_openmp_thread_number: 1 NCO: "4.6.0" history: Tue Dec 27 14:15:22 2016: ncatted -a dimension... In [5]: import cftime

In [6]: cftime.version
Out[6]: '1.0.3.4'

In [7]: xr.version
Out[7]: '0.14.0' ```