Change import of collections types.

[rpgoldman]

I was getting this warning:

Warning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working
    from collections import MutableSequence, Iterable

-- Docs: https://docs.pytest.org/en/stable/warnings.html

[tcmitchell]

Thanks for the pull request. Can you say a bit more about what you were doing that allowed you to see this warning? Were you using pytest instead of Python's unittest? Anything else? I'd like to be able to recreate it if possible so that we can maybe add a unit test.

[rpgoldman]

I'm pretty sure I was doing pytest, since unittest is above my pay grade ...

[tcmitchell]

Thanks for locating this issue and supplying the fix. There are more collections warnings lurking which will be covered by #274