Add type annotations

[bhrutledge]

I've been working on adding type annotations to Twine (pypa/twine#231), and keyring is one of our imports that's missing type hints/stubs. Is there any interest in adding those, either in the source or to https://github.com/python/typeshed?

See also: https://mypy.readthedocs.io/en/stable/existing_code.html

[jaraco]

I'm happy for someone to contribute type annotations. This codebase has mypy support, so anyone who would like to add annotations, especially ones that add meaningful value, I'm all for it.

[jaraco]

The project has one typing annotation and welcomes more. mypy tests pass and the project now includes a py.typed file.