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
677307460,MDU6SXNzdWU2NzczMDc0NjA=,4337,cftime_range does not support default cftime.datetime formatted output strings,6063709,closed,0,,,5,2020-08-12T01:28:30Z,2020-08-17T23:27:07Z,2020-08-17T23:27:07Z,CONTRIBUTOR,,,,"<!-- Please do a quick search of existing issues to make sure that this has not been asked before. -->

**Is your feature request related to a problem? Please describe.**

The `xarray.cftime_range` does not support datetime strings that are the default output from `cftime.datetime.strftime()` which are the format which `cftime_range` itself uses internally.

```python
import cftime
import xarray
date = cftime.datetime(10,1,1).strftime()
print(date)
xarray.cftime_range(date, periods=3, freq='Y')
```
outputs
```
10-01-01 00:00:00
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-70-a16c1fcab8d6> in <module>
      3 date = cftime.datetime(10,1,1).strftime()
      4 print(date)
----> 5 xarray.cftime_range(date, periods=3, freq='Y')

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in cftime_range(start, end, periods, freq, normalize, name, closed, calendar)
    963 
    964     if start is not None:
--> 965         start = to_cftime_datetime(start, calendar)
    966         start = _maybe_normalize_date(start, normalize)
    967     if end is not None:

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in to_cftime_datetime(date_str_or_date, calendar)
    683                 ""a calendar type must be provided""
    684             )
--> 685         date, _ = _parse_iso8601_with_reso(get_date_type(calendar), date_str_or_date)
    686         return date
    687     elif isinstance(date_str_or_date, cftime.datetime):

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _parse_iso8601_with_reso(date_type, timestr)
    101 
    102     default = date_type(1, 1, 1)
--> 103     result = parse_iso8601(timestr)
    104     replace = {}
    105 

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in parse_iso8601(datetime_string)
     94         if match:
     95             return match.groupdict()
---> 96     raise ValueError(""no ISO-8601 match for string: %s"" % datetime_string)
     97 
     98 

ValueError: no ISO-8601 match for string:   10-01-01 00:00:00
```

**Describe the solution you'd like**
It would be good if `xarray.cftime_range` supported the default `strftime` format output from cftime.datetime objects. It is confusing that it uses this format with `repr` but explicitly does not support it.

**Describe alternatives you've considered**

Specifying an ISO-8601 compatible format (using `T` separator) isn't general as it doesn't work for years < 1000 because the year field is not zero padded.
```python
import cftime
import xarray
date = cftime.datetime(10,1,1).strftime('%Y-%m-%dT%H:%M:%S')
print('|{}|'.format(date))
xarray.cftime_range(date, periods=3, freq='Y')
```
produces
```
|  10-01-01T00:00:00|
```
and the error as above.

A work-around is to zero-pad manually
```python
import cftime
import xarray
date = '{:0>19}'.format(cftime.datetime(10,1,1).strftime('%Y-%m-%dT%H:%M:%S').lstrip())
print(date)
xarray.cftime_range(date, periods=3, freq='Y')
```
produces
```
0010-01-01T00:00:00
CFTimeIndex([0010-12-31 00:00:00, 0011-12-31 00:00:00, 0012-12-31 00:00:00], dtype='object')
```

**Additional context**
I think this is a relatively small addition to the codebase but would make it easier and less confusing to use the default format that is also used by the the function itself. It is easy to support as it is consistent and uniform.
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