Move over to using poetry for packaging and pre-commit for code checking

This MR is big but largely mechanical.

The first commit moves from using setup.py over to using poetry for packaging. This aligns better with our current Python project guidelines. No package version specifiers have been changed from the original requirements.txt list and the resulting package should be identical.

The second commit adds our .pre-commit-config.yaml file and fixes up the code base via black and isort. No functionality changes are present but this did catch a type annotation bug with the override_permissions_spec decorator.

The third commit updates the README and CHANGELOG.

Closes #3 (closed)