python: Specify the JSON separators

python: Specify the JSON separators

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>