pynio causes dependency conflicts in py36 CI build

On Saturday night, all Python 3.6 CI builds started failing. Python 3.7 is unaffected.
See https://dev.azure.com/xarray/xarray/_build/results?buildId=362&view=logs

MacOSX py36:

UnsatisfiableError: The following specifications were found to be in conflict:
  - pynio
  - python=3.6
  - rasterio

Linux py36:

UnsatisfiableError: The following specifications were found to be in conflict:
  - cfgrib[version='>=0.9.2']
  - h5netcdf
  - pynio

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