Use PEP-0561 to indicate to library users that type hints are available

[mathieui]

Hello,

aiodns contains type hints, but code that uses aiodns does not benefit from having it typed, due to the lack of a "py.typed" file in the module root. This should be easily fixed by creating the file and adding it as package data in the setup script.

ref: https://www.python.org/dev/peps/pep-0561/