Import ABC from collections.abc for Python 3.10 compatibility.

[tirkarthi]

Fixes #83

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[jtniehof]

Thanks for taking a shot at this. Normally we try to preserve namespacing per PEP 20 (this isn't consistent throughout the codebase) and we definitely do need some tests to cover this. (I'll probably pull over the warnings testing infrastructure from SpacePy).

[dnadeau-lanl]

Fixed in #120

[jtniehof]

Superseded by #120 for the reasons described in that PR.