Bug#913521: python-xarray: Needs python3.6 for build tests but doesn't Build-Depend on it

[Jeremy Bicha]

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