Remove Python 2 compatibility

[bessbd]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md) or test/build only changes
• [x] Completed the PR template below:

Description

Python 2 was EoLed back in January. This commit removes it from the list of supported environments.

Approach

Schema & API Changes

Security and Privacy

Testing

Monitoring and Logging

[bessbd]

Updates also needed here: https://github.com/cloudant/python-cloudant/blob/d080687766a3101b1e876957674983b956851a76/setup.py#L57

I'm not sure if PyPi does anything to prevent installs on 2 if that ^ information changes. Given 2.14 will work with python 2 because of #477 - then I guess this change should potentially only merge after that release, such that the tagged README etc for 2.14 matches it's capability and then everything on master after that does not guarantee compatibility. Does that make sense?

Done in 7182ef4

Holding off with this makes sense.

[bessbd]

Thank you for the review, @ricellis and @emlaver . I'm about to merge this.