Switch package management to Poetry

[arnavb]

Pipenv has a lot of issues.

• Namely, it isn't actively maintained anymore (pypa/pipenv#4058); no commits, release, or active responses to issues or PRs.
• It just isn't working sometimes. Right now it isn't respecting platform specific markers for subdependencies (atomicwrites in particular).

That's why I'm considering moving this project to Poetry, an active project that seems to avoid many of Pipenv's warts. Of course, many things need to be migrated:

• The entire packaging system needs to be rewritten (Using the same minimum versions in setup.py).
• Travis CI deploys need to be rewritten.