Comply with PEP 561

[ppfeister]

Add type hints for exposed sherlock function, add py.typed to comply with PEP 561 and allow for detection by mypy for static type checking.

[ppfeister]

Odd. I figured Optional would be available without typing since other types like (lowercase) dict and (lowercase) list were brought into the std library, negating typing for many of those. Seems that it needs the import still.

Sometime tomorrow, I'll either add the import or switch it to the normal union without an import (str | None).