Migrate deps and build to poetry

Had a hard time continuing to use pipenv for dependency management
while working with readthedocs automated builds. Migrated over to
Poetry and its PEP517 compliance will allow for easier use with
readthedocs and less overhead overall.

Here are the sources I used to help make this decision:
* python-poetry/poetry#1145
* python-poetry/poetry#1941
* readthedocs/readthedocs.org#3181