Use pyproject.toml

[kstrauser]

Python packaging is moving toward using pyproject.toml to replace setup.py, requirements.txt, and other related files. This PR makes that change by:

• Using Poetry to create a new pyproject.toml
• Moving test-requirements.txt into the same file
• Moving MANIFEST.in into the same file
• Updating tox to use poetry to build the test environments
• Replacing the old mock development dependency with the built-in unittest.mock while mucking around with all of it (one less dependency!)

This builds on #619 so it's currently based off that PR. Once 619 is merged this should be rebased off that branch.