Mathieu Bridon authored Jun 27, 2018 and Eric Engestrom committed Jul 05, 2018

On Python 2, the default JSON separators are ', ' for items and ': ' for
dicts.

On Python 3, the default is the same when no indent is specified, but if
one is (and we do specify one) then the default items separator becomes
',' (the dict separator remains unchanged).

This change explicitly specifies the Python 3 default, which helps
ensuring that the output is identical, whether it was generated by
Python 2 or 3.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>