Type hints & PEP 561 packaging

[tuukkamustonen]

Add type hints to all (public API) code make the distribution PEP 561 compliant. This allows mypy (and other tools?) to find the type hints and use them in linting.

In practice, add py.typed file to statsd package, and include it in the package:

setup(
    package_data={'statsd': ['py.typed']},
    zip_safe=False,  # not needed with wheels, AFAIK
)

PEP 561: https://www.python.org/dev/peps/pep-0561/

[orsinium]

I want to give it a try but with dropping support for Python 2.7. Writing all type annotations as stubs or comments is a bit painful and hard to maintain.

[jsocol]

Python 2.7 is officially dropped!