Use Python 3.6 on CI

[tisnik]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@d624840). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #114   +/-   ##
=========================================
  Coverage          ?   95.46%           
=========================================
  Files             ?       21           
  Lines             ?      838           
  Branches          ?        0           
=========================================
  Hits              ?      800           
  Misses            ?       38           
  Partials          ?        0

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[centos-ci]

@tisnik Your image is available in the registry: docker pull MISSING_REGISTRY_NAME/MISSING_IMAGE_NAME:SNAPSHOT-PR-114