test against xarray HEAD

[dcherian]

Testing against Xarray HEAD would've caught pydata/xarray#5329 a month or more ago. Would it be possible to set that up? It has usually helped catch a lot of pre-release bugs.

[forman]

@dcherian I agree that this would help finding issues with xarray early, but currently this will produce to much overhead for us. We may think about about a dedicated branch that we'll keep up-to-date with our master with the difference that we use xarray HEAD dependency.