xarray fails in Python 3.6 now (observed by hklpy's CI workflow)

[prjemian]

In the CI workflow for https://github.com/bluesky/hklpy, failures began this week when testing the Python 3.6 build. These failures did not occur in the PR, but only appeared after two PRs were merged.

Looking here, xarray is required but without version restriction. Shall we consider dropping Python 3.6 here due to this failure? That's the conclusion in hklpy.