Fix file output format from dict repr to json.dumps

[prog893]

str() method converts dict to string with dict representation (__repr__), which is not valid JSON, but valid Python code.

Since we only care about JSON logs anyways, this PR fixes string conversion with json.dumps.

[toVersus]

Ah, gotcha. LGTM

>>> dict = {"name": "john"}
>>> type(dict)
<class 'dict'>
>>> str(dict)
"{'name': 'john'}" # invalid json format
>>> json.dumps(dict)
'{"name": "john"}' # valid json format