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

Author: Joe Hamman

ci/requirements-py36.yml

@@ -20,14 +20,14 @@ dependencies:
   - scipy
   - seaborn
   - toolz
-  # - rasterio  # xref #2683
+  - rasterio
   - bottleneck
   - zarr
   - pseudonetcdf>=3.0.1
   - eccodes
   - cdms2
-# - pynio  # xref #2683
-# - iris>=1.10  # xref #2683
+  - pynio
+  - iris>=1.10
   - pydap
   - lxml
   - pip: