Build failure with pandas master

[shoyer]

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 pandas-dev/pandas#29062, which adds a .attrs attribute to pandas objects. We copy this attribute in the DataArray constructor.