Python 3.9 removed a couple of deprecated functions/arguments that we

used, which was the only thing preventing us from claiming Python 3.9

support.
This change updates our usage of json.loads() to not pass an

encoding= argument (removed in Python 3.9) and to instead handle

decoding byte strings up-front, and to update unit tests to not depend

on an internal, deprecated method on HTMLParser.
setup.py has been updated to reflect the current supported list of

Python versions.

Unit tests pass.