I just noticed that pandas 0.23.3-1 appears to have fixed the issue that led to the build tests only being run for python3.6. So I suggest dropping the special version handling in python-xarray. Maybe next time you can ask the debian-wb-team to rebuild pandas whenever there is a new supported python3 version.
Note that there is a newutopkgtest failure (but not a build test failure) at https://bugs.debian.org/913524 Thanks, Jeremy Bicha
