Type error in diversity and Tajimas_D with xarray 0.19.0

xarray 0.19.0 raises a type error when using a DataArray object to construct a variable: https://github.com/pydata/xarray/pull/5630 [(line)](https://github.com/pydata/xarray/pull/5630/files#diff-43c76e9be8425b5b6897dcecab4b240c32580447455c0c8c0b9b7fd84ce8a15dL119).
This is affecting sgkits `diversity` and `Tajimas_D` methods (tests failing).