+ upgrade to use collections.abc as needed for python 3.6+

[slorg1]

Addresses #72

[slorg1]

@AndreaCensi kindly, please review this and, if you approve, let's merge it in to keep being compatible with Python 3.6+

[CoconutTherapy]

Greatings, could we have this merged soon?

[feanil]

@AndreaCensi any chance this could get merged, it's not yet blocking our upgrade to Python 3.9 but will be soon.

[aleehedl]

Bumping this @AndreaCensi.