Add py.typed

[Goldziher]

Hi,

This library doesn't have a py.typed file, and hence considered untyped by mypy and other checkers.

Solution:

add a py.typed file as part of the package. This is a small thing.

Here is the pertinent info: https://peps.python.org/pep-0561/

[kurtmckee]

This is a duplicate of #369.