Import ABC from collections.abc for Python 3.10 compatibility.

[tirkarthi]

Ref : python/cpython#23754

ABC aliases were removed from collections in favor of collections.abc

[hugovk]

👍

Between Python 3.3 and 3.9, Mapping was in both collections and collections.abc

But in Python 3.10 it was removed from collections and is only available in collections.abc.

Because only 3.6 and up is supported here, this is change is safe to make for 3.6-3.9, and is needed for 3.10 and up.

[0xdeaddood]

Thanks @tirkarthi for the PR and @hugovk for checking.