Restrict tests to python bookend versions (#6437)
Restrict tests to python bookend versions (#6437)

This speeds up CI, and it would be quite unlikely to fail _only_ a middle version
pydata/xarray,pydata/xarray,markelg/xray,pydata/xarray,jhamman/xarray,jhamman/xarray,jhamman/xarray,markelg/xray,markelg/xray