Support Python 3 versions of JSON decode errors.
Review Request #10488 — Created April 1, 2019 and submitted
Python 3 enhanced the error messages when encountering JSON content that
can't be decoded. This change fixes up a couple unit tests that compare
against these error messages, checking the appropriate version depending
on the version of Python.
Unit tests pass on Python 2.7 with Django 1.6 and Python 3.5-3.7 with
Django 1.11 (with other in-progress changes).