Drop CI testing of Python 2.7 and 3.6

[kurtmckee]

Description

Drop CI testing for Python 2.7 and Python 3.6, which have both reached end-of-life status.

In addition, this adds a CHANGELOG, which is critical file to have for other developers' understanding of the project.

Pre-approval checklist (for submitter)

Please complete these steps

• Passes tests
• New tests for additional features or changed functionality
• My name and contribution added to contributors list (or if I'd rather opt out, I've said so in the PR)

I don't want my name added to the contributors list for such a small change.

Note that the lint check is failing in CI due to existing failures in the dependencies. I'll try to fix this in a separate PR.