Use collections.abc for Python 3.9 compatibility.

[tirkarthi]

• Importing ABC from collections module was deprecated and removed in Python 3.9 . Use collections.abc instead for Python 3.9 compatibility. Reference : bpo-25988: Do not expose abstract collection classes in the collections module. python/cpython#10596

[sirosen]

Yeah, this seems reasonable.
I'll see about adding 3.9 as an "allow failures" part of the build matrix at some point. Our Travis setup needs a bit of a refresh.