Migrate project to Python 3 and add testing with CI

[kevincon]

I have heard from a few people that it has become difficult to reproduce our original results due to the project's Python dependency versions no longer being available on pypi and newer versions of those dependencies breaking API compatibility, so this PR does the following to help protect the project's reproducability into the future:

• Migrates the project to Python 3
• Updates the project to use the latest version of most of the project's dependencies
• Replaces the requirements.txt file with pipenv
• Adds pytest-based tests to capture the project's important results
• Facilitates the execution of the tests using tox
• Automates testing the project via Travis CI
• Updates the README for these changes