Add missing py.typed

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This project is typed, but the typing cannot be used due to a missing py.typed file.

It’s literally just an empty file named py.typed in the project root, included in the package.

https://mypy.readthedocs.io/en/stable/installed_packages.html

[markedwards]

If I make a PR, will you release it?

[markedwards]

Any idea of timing on a release that includes this change? Thanks.