MWV of travis build

[kylepjohnson]

More to follow up on. Examples using poetry to look at more:

• https://wrongsideofmemphis.com/2018/10/28/package-and-deploy-a-python-module-in-pypi-with-poetry-tox-and-travis/
• Poetry and CI builds -- best practices, knowledge share (travisci), and discussion python-poetry/poetry#366
• Suggested workflow for testing a build on a CI service? python-poetry/poetry#166

I'm using tox in the Makefile but for travis multiple environments are declared differently. Need to figure that out, too.