Add py.typed file to mark package PEP 561-compatible

[cpnielsen]

Description

Looking at the code, it seems most (all?) of it is fully typed. Unfortunately, mypy and potentially other type checkers does not consider it typed unless it includes a py.typed file at the root of the package.

See PEP 561 for details.

I am happy to provide a PR if needed, but it seems like a straight-forward change (if desired).