Type hints & PEP 561 packaging

[tuukkamustonen]

Add type hints to all (at least 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 an empty awscrt/py.typed file, and include it in the package:

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

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