require higher xarray version

[lindnemi]

fixes an error that occurs in pypsa ariadne

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9b0c47f) 89.28% compared to head (1b9b467) 89.28%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #218 +/- ## ======================================= Coverage 89.28% 89.28% ======================================= Files 15 15 Lines 3359 3359 Branches 773 773 ======================================= Hits 2999 2999 Misses 248 248 Partials 112 112 ```

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[FabianHofmann]

thanks @lindnemi !