reintroduce pynio/rasterio/iris to py36 test env (#2738)
reintroduce pynio/rasterio/iris to py36 test env (#2738)

* add h5netcdf+dask tests

* pep8

* reactivate pynio/rasterio/iris in py36 test builds

* revert changes to test_backends.py -- unrelated to this PR

chunweiyuan/xarray,jhamman/xarray,markelg/xray,shoyer/xray,shoyer/xarray,pydata/xarray,jhamman/xarray,shoyer/xarray,pydata/xarray,markelg/xray,jhamman/xarray,markelg/xray,jhamman/xray,pydata/xarray,xray/xray