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.

contrib: require Python 3.6 for byteify-strings.py This script makes use of `token.COMMENT`, which apparently isn't present until Python 3.6. So make the script and its test conditional on Python 3.6.

demandimportpy3: only use lazy extension loader on Python 3.6+ There was an inline comment denoting a bug in the lazy extension loader on Python 3.5 which prevents it from working there. But the code was not conditional on the Python version. The result of this was a myriad of failures on Python 3.5 due to getattr() and friends not working on lazy extension modules. By making extension modules non-lazy on Python 3.5, we reduce the number of test failures from 48 to 22 on that Python version.