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
798592803,MDU6SXNzdWU3OTg1OTI4MDM=,4853,cftime default's datetime breaks CFTimeIndex,20629530,open,0,,,4,2021-02-01T18:14:21Z,2021-02-05T18:48:31Z,,CONTRIBUTOR,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**What happened**:
With `cftime` 1.2.0, one can create datetime object with `cftime.datetime(*args, calendar='calendar')`, instead of using one of the subclasses (ex `cftime.DatetimeNoLeap(*args)`). In the latest release (1.4.0, yesterday), the subclasses have been deprecated, but kept as legacy. While all xr code still works (it is using the legacy subclasses), the `CFTimeIndex` object relies on the _type_ of the datetime object in order to infer the calendar. If the datetime was created outside xarray, using the now default constructor, the returned type is not understood and `CFTimeIndex`breaks.

**What you expected to happen**:
I expected `CFTimeIndex` to be independent of the way the datetime object is created.

**Minimal Complete Verifiable Example**:

```python3
import cftime
import numpy as np
import xarray as xr

# A datetime array, not created in xarray
time = cftime.num2date(np.arange(365), ""days since 2000-01-01"", calendar=""noleap"")
a = xr.DataArray(np.zeros(365), dims=('time',), coords={'time': time})

a.indexes['time']
```
Fails with : 
```python3
Traceback (most recent call last):
  File ""<stdin>"", line 1, in <module>
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/cftimeindex.py"", line 342, in __repr__
    attrs_str = format_attrs(self)
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/cftimeindex.py"", line 264, in format_attrs
    attrs[""freq""] = f""'{index.freq}'"" if len(index) >= 3 else None
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/cftimeindex.py"", line 692, in freq
    return infer_freq(self)
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/frequencies.py"", line 96, in infer_freq
    inferer = _CFTimeFrequencyInferer(index)
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/frequencies.py"", line 105, in __init__
    self.values = index.asi8
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/cftimeindex.py"", line 673, in asi8
    [
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/coding/cftimeindex.py"", line 674, in <listcomp>
    _total_microseconds(exact_cftime_datetime_difference(epoch, date))
  File ""/home/phobos/Python/xclim/.tox/py38/lib/python3.8/site-packages/xarray/core/resample_cftime.py"", line 370, in exact_cftime_datetime_difference
    seconds = b.replace(microsecond=0) - a.replace(microsecond=0)
  File ""src/cftime/_cftime.pyx"", line 1153, in cftime._cftime.datetime.__sub__
ValueError: cannot compute the time difference between dates with different calendars
```
**Anything else we need to know?**:

**Environment**:

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

<!-- Paste the output here xr.show_versions() here -->
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.5 | packaged by conda-forge | (default, Jul 31 2020, 02:39:48) 
[GCC 7.5.0]
python-bits: 64
OS: Linux
OS-release: 5.10.11-arch1-1
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: fr_CA.utf8
LOCALE: fr_CA.UTF-8
libhdf5: 1.12.0
libnetcdf: 4.7.4

xarray: 0.16.2
pandas: 1.2.1
numpy: 1.20.0
scipy: 1.6.0
netCDF4: 1.5.5.1
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.4.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2021.01.1
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: 0.16.1
setuptools: 46.1.3
pip: 20.1
conda: None
pytest: 6.2.2
IPython: 7.19.0
sphinx: None

</details>
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