Include py.typed marker in pypi packages

[OliverHofkens]

First off, thanks for this great project!

It's great that this project has extensive typing support and a py.typed marker. The only missing part is that the marker is not included in the distributed package.

Ref: https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages

I'll happily open a PR to include it!