Importing ABC directly from collections module was removed in Python 3.9

[tirkarthi]

https://bugzilla.redhat.com/show_bug.cgi?id=1792921

https://github.com/dmeranda/demjson/blob/5bc65974e7141746acc88c581f5d2dfb8ea14064/test/test_demjson.py#L118

Thanks for the library!