py3: define and use json.loads polyfill...
py3: define and use json.loads polyfill
Python 3.5's json.loads() requires a str. Only Python 3.6+
supports passing a bytes or bytearray.
This commit implements a json.loads() polyfill on Python 3.5
so that we can use bytes. The added function to detect encodings
comes verbatim from Python 3.7.
