[issue-228] make type hints in code readable for downstream packages

[meretp]

We need to add a py.typed file to the source directory of the package so that other downstream tools can use the type hints.

For reference:

• https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages
• https://jugmac00.github.io/blog/bite-my-shiny-type-annotated-library/#poetry