Add py.typed to package

[akshayphdk]

I'm trying to use python-json-logger in my project, but mypy is surfacing this error:

error: Skipping analyzing "pythonjsonlogger": module is installed, but missing library stubs or py.typed marker
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

I believe there is an open issue related to this: #118
My guess why this continues to happen even though py.typed exists in the repo is because it isn't added to the package.

[bringhurst]

Just fyi, this marker was intentionally excluded in #129 since typing was incomplete at the time.

Before this is merged in, it would probably be a good idea to set disallow_untyped_defs = true in setup.cfg and fix anything it complains about.

[madzak]

Thanks, will evaluate bringhursts note after I'm done merging pull requests.

[mike-oakley]

@madzak any chance you could cut a release which includes this fix? 🙏🏼