issues

• [x] Closes #3161
• [x] Tests added
• [x] Passes black . && mypy . && flake8
• [x] Fully documented, including whats-new.rst for all changes and api.rst for new API

``` __ TestNetCDF3ViaNetCDF4Data.test_roundtrip_cftime_datetime_data ___

dates = array([cftime.DatetimeProlepticGregorian(0001-01-01 00:00:00), cftime.DatetimeProlepticGregorian(0001-01-02 00:00:00)], dtype=object) units = 'days since 0001-01-01 00:00:00.000000' calendar = 'proleptic_gregorian'

def encode_cf_datetime(dates, units=None, calendar=None):
    """Given an array of datetime objects, returns the tuple `(num, units,
    calendar)` suitable for a CF compliant time variable.

    Unlike `date2num`, this function can handle datetime64 arrays.

    See also
    --------
    cftime.date2num
    """
    dates = np.asarray(dates)

    if units is None:
        units = infer_datetime_units(dates)
    else:
        units = _cleanup_netcdf_time_units(units)

    if calendar is None:
        calendar = infer_calendar_name(dates)

    delta, ref_date = _unpack_netcdf_time_units(units)
    try:
        if calendar not in _STANDARD_CALENDARS or dates.dtype.kind == "O":
            # parse with cftime instead

          raise OutOfBoundsDatetime

E pandas._libs.tslibs.np_datetime.OutOfBoundsDatetime

xarray/coding/times.py:400: OutOfBoundsDatetime

During handling of the above exception, another exception occurred:

self = <xarray.tests.test_backends.TestNetCDF3ViaNetCDF4Data object at 0x7fd54f4cb780>

@requires_cftime
def test_roundtrip_cftime_datetime_data(self):
    from .test_coding_times import _all_cftime_date_types

    date_types = _all_cftime_date_types()
    for date_type in date_types.values():
        times = [date_type(1, 1, 1), date_type(1, 1, 2)]
        expected = Dataset({"t": ("t", times), "t0": times[0]})
        kwargs = {"encoding": {"t0": {"units": "days since 0001-01-01"}}}
        expected_decoded_t = np.array(times)
        expected_decoded_t0 = np.array([date_type(1, 1, 1)])
        expected_calendar = times[0].calendar

        with warnings.catch_warnings():
            if expected_calendar in {"proleptic_gregorian", "gregorian"}:
                warnings.filterwarnings("ignore", "Unable to decode time axis")

          with self.roundtrip(expected, save_kwargs=kwargs) as actual:

xarray/tests/test_backends.py:483:

/usr/share/miniconda/envs/xarray-tests/lib/python3.6/contextlib.py:81: in enter return next(self.gen) xarray/tests/test_backends.py:243: in roundtrip self.save(data, path, save_kwargs) xarray/tests/test_backends.py:265: in save path, engine=self.engine, format=self.file_format, kwargs xarray/core/dataset.py:1536: in to_netcdf invalid_netcdf=invalid_netcdf, xarray/backends/api.py:1071: in to_netcdf dataset, store, writer, encoding=encoding, unlimited_dims=unlimited_dims xarray/backends/api.py:1117: in dump_to_store store.store(variables, attrs, check_encoding, writer, unlimited_dims=unlimited_dims) xarray/backends/common.py:293: in store variables, attributes = self.encode(variables, attributes) xarray/backends/common.py:382: in encode variables, attributes = cf_encoder(variables, attributes) xarray/conventions.py:748: in cf_encoder new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()} xarray/conventions.py:748: in <dictcomp> new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()} xarray/conventions.py:250: in encode_cf_variable var = coder.encode(var, name=name) xarray/coding/times.py:446: in encode data, encoding.pop("units", None), encoding.pop("calendar", None) xarray/coding/times.py:419: in encode_cf_datetime num = _encode_datetime_with_cftime(dates, units, calendar) xarray/coding/times.py:366: in _encode_datetime_with_cftime return np.vectorize(encode_datetime)(dates) /usr/share/miniconda/envs/xarray-tests/lib/python3.6/site-packages/numpy/lib/function_base.py:2755: in call return self._vectorize_call(func=func, args=vargs) /usr/share/miniconda/envs/xarray-tests/lib/python3.6/site-packages/numpy/lib/function_base.py:2825: in _vectorize_call ufunc, otypes = self._get_ufunc_and_otypes(func=func, args=args) /usr/share/miniconda/envs/xarray-tests/lib/python3.6/site-packages/numpy/lib/function_base.py:2785: in _get_ufunc_and_otypes outputs = func(*inputs) xarray/coding/times.py:364: in encode_datetime return np.nan if d is None else cftime.date2num(d, units, calendar)

??? E AttributeError: 'cftime._cftime.DatetimeProlepticGregorian' object has no attribute 'tzinfo'

cftime/_cftime.pyx:186: AttributeError ```

temporarily pin cftime so that we can keep merging.