Add type hints for public APIs

[hynek]

I'm too busy lazy to add stubs for everything, but this should work for most people without resorting to Mypy plugins etc?

[playpauseandstop]

Only advice I can give is covering environ.bool_var(...) call within typing_examples module. Beside that all looks great! Thanks a lot for your time

[hynek]

Yeah thanks! And ugh, I forgot about the whole secrets module, too.