Add isort dependency and mypy.ini

Relates to: #274 (closed)

• isort was used in the CI template but wasnt installed as a dev dependency
• mypy has errors out of the box since backoff doesnt have type hints yet. Its in prerelease https://github.com/litl/backoff/releases/tag/v2.0.0-alpha now but I think its a subdependency for us so we wont be able to upgrade quickly
• mypy needs a mypy.ini file since it doesnt support tox configs as of now