Import ABC from collections.abc instead of collections for Python 3 compatibility

[tirkarthi]

Fixes #23

[rapgro]

This patch seems to not work with Python 3.4 or prior, but 3.5 and afterwards, see checks failed in CI.

[tirkarthi]

The errors seem unrelated to the PR changes. It seems pytest 5.x is pulled in for Python 3.4 too which is not supported : https://docs.pytest.org/en/6.2.x/py27-py34-deprecation.html . Either pytest 4.x series can be pinned to ensure pytest works on Python 3.4 or the typing module can be added as a dependency for python version 3.4 and below.