Bug#904785: python-xarray autopkg tests fail on arm64

[Matthias Klose]

Package: src:python-xarray
Version: 0.10.8-1

the python-xarray autopkg tests fail on arm64, as seen in
https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-cosmic/cosmic/arm64/p/python-xarray/20180728_004139_2ff97@/log.gz

I assume you can reproduce these when building the package on an arm64 porter box.