Actually add the PEP 561 typed module marker.

[ppentchev]

Hi,

Thanks a lot for writing and maintaining this excellent promise module!

When adding promise support to a module that I'm still writing, I found out that although in version 2.2 you added the py.typed marker file to setup.py's package_data dictionary, it does not actually exist, so it is not really distributed and the mypy checker did not believe me that your module was really fully typed :) What do you think about this trivial commit that fixes that?

Thanks again, and keep up the great work!

G'luck,
Peter