Make setup.py compatible with python 2

[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

This fixes https://github.com/cloudant/python-cloudant/issues/476

Approach

See related https://stackoverflow.com/a/10971047/5323166

Schema & API Changes

Security and Privacy

Testing

Monitoring and Logging

[bessbd]

It's worth mentioning that I was not able to test this change as I couldn't install a python 2 on my machine: https://github.com/Homebrew/homebrew-core/pull/49796

[emlaver]

@bessbd I created a branch that tests Python 2 in Travis and Jenkins. Both look to be passing: https://travis-ci.org/github/cloudant/python-cloudant/builds/717927835 https://cloudant-sdks-jenkins.swg-devops.com/blue/organizations/jenkins/Main%2Fpython-cloudant/detail/test-python-2/1/pipeline/

[bessbd]

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