Temporarily remove pynio from py36 CI build (#3157)

This should get things building again.

Author: shoyer

ci/requirements/py36.yml

@@ -27,7 +27,9 @@ dependencies:
   - pseudonetcdf>=3.0.1
   - cfgrib>=0.9.2
   - cdms2
-  - pynio
+  # Disabled due to conflicting dependencies:
+  # https://github.com/pydata/xarray/issues/3154
+  # - pynio
   - iris>=1.10
   - pydap
   - lxml