StructlogFormatter conforms to structlog.typing.Processor

[AWhetter]

This changes the function signature of StructlogProcessor.__call__ so that's it's compatible with structlog.typing.Processor.

I've also removed the need for mypy to be run with --no-warn-unused-ignores. The test was requiring this to be set, and therefore users would need to as well. With this change, users can run mypy with --strict and without needing to turn any other checks off.

Closes #146