Add type hints

[pietermarsman]

I use mypy and want to have type hints for retrying to fix this error:

error: Skipping analyzing "retrying": module is installed, but missing library stubs or py.typed marker  [import]
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

Current non-optimal solution is to ignore by adding the following lines to the mypy.ini config.

[mypy-retrying.*]
ignore_missing_imports = True