Annotate types for Actor and Message

[orsinium]

1. Fix existing type violations reported by mypy.
2. Annotate types for Actor, actor, and Message
3. Use TypeVar with Generic to propagate the return type of the function. For example, if you wrap a function that returns str, then mypy will know that calling Actor instance also returns str.

Something that can be improved later:

1. Message is a namedtuple, and so cannot inherit from Generic. It can be solved by using a dataclass instead.
2. Types for arguments of the function can be typed with Parameter Specification Variables, but for that, you'll need typing-extensions the good news is that it can be static-only dependency by putting it inside of if TYPE_CHECKING. But I didn't want to complicate this PR too much.

[Bogdanp]

Thanks!