Add py.typed marker

[smheidrich]

Running Mypy on packages that depend on this package, one gets errors like:

example.py:1: error: Skipping analyzing "renameat2": module is installed, but missing library stubs or py.typed marker  [import]
example.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

As the error message suggests, this can be solved by adding a py.typed marker to the package data files, which is what this PR does.

[jordemort]

Thanks, LGTM. Looks like I need to brush some dust off the CI here. I'll try and get to that this week and then kick out a release with your change.