Build failure with pandas master

See https://dev.azure.com/xarray/d5e7a686-a114-4b8c-a2d8-4b5b11efd896/_build/results?buildId=1218&view=logs&jobId=41d90575-019f-5cfd-d78e-c2adebf9a30b for a log.

Appears to be due to https://github.com/pandas-dev/pandas/pull/29062, which adds a `.attrs` attribute to pandas objects. We copy this attribute in the DataArray constructor.