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

[cla-checker-service]

💚 CLA has been signed

[AWhetter]

Is there anything that I can do to help move this forwards?

[AWhetter]

I've updated this to address the latest merge conflicts, so this is ready to review again.

[xrmx]

We run mypy in pre-commit, so instead of adding these tests and adding mypy as dev dependency we should update the mypy configuration in .pre-commit-config.yaml.

[xrmx]

Sorry missed that the purpose of the typing test are testing the example. I guess that's fine then.

[AWhetter]

I implemented it this way based on your comment here, but I wasn't sure if I'm interpreting what you meant correctly.
I could move the running of mypy into the "test" nox session if you prefer? Or I could separate the running of mypy out into a new nox session and execute that in the github actions?