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
728888328,MDExOlB1bGxSZXF1ZXN0NTA5NTAyMjA2,4534,Add isocalendar to dt fields,5635139,closed,0,,,17,2020-10-24T22:13:29Z,2020-11-19T23:36:20Z,2020-11-19T08:04:16Z,MEMBER,,0,pydata/xarray/pulls/4534,"<!-- Feel free to remove check-list items aren't relevant to your change -->

 - [x] Tests added
 - [x] Passes `isort . && black . && mypy . && flake8`
 - [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
 - [x] New functions/methods are listed in `api.rst`

This currently fails tests — IIUC because our infra for this expects scalars rather than tuples, and `isocalendar` returns a tuple.

The existing `week` & `weekofyear` are going to stop working at some point, so if we can move off those that would be ideal:

```
xarray/tests/test_accessor_dt.py::TestDatetimeAccessor::test_field_access[weekofyear]
xarray/tests/test_accessor_dt.py::TestDatetimeAccessor::test_field_access[week]
  /Users/maximilian/workspace/xarray/xarray/tests/test_accessor_dt.py:72: FutureWarning: weekofyear and week have been deprecated, please use DatetimeIndex.isocalendar().week instead, which returns a Series.  To exactly reproduce the behavior of week and weekofyear and return an Index, you may call pd.Int64Index(idx.isocalendar().week)
    getattr(self.times, field), name=field, coords=[self.times], dims=[""time""]

xarray/tests/test_accessor_dt.py::TestDatetimeAccessor::test_field_access[weekofyear]
xarray/tests/test_accessor_dt.py::TestDatetimeAccessor::test_field_access[week]
  /Users/maximilian/workspace/xarray/xarray/core/accessor_dt.py:44: FutureWarning: Series.dt.weekofyear and Series.dt.week have been deprecated.  Please use Series.dt.isocalendar().week instead.
    field_values = getattr(values_as_series.dt, name).values

-- Docs: https://docs.pytest.org/en/stable/warnings.html
```

I'd be very happy for someone else to take this on...","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4534/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull