Fix #795: Fix typing of actor decorator to work with async actors

[janek-cosmose]

No description provided.

[janek-cosmose]

Fixes #795

[janek-cosmose]

Tested with mypy 1.8.2 on dramatiq project and on sample file:

import dramatiq


@dramatiq.actor
async def foo() -> None:
    pass


@dramatiq.actor()
async def foo1() -> None:
    pass


@dramatiq.actor
def foo2() -> None:
    pass


@dramatiq.actor()
def foo3() -> None:
    pass

[LincolnPuzey]

@janek-cosmose Thanks for the PR! Looks good to me.

Can you add yourself to CONTRIBUTORS.md, and also add E704 to the ignored errors in .flake8 since it is conflicting with black.

Then I think we can merge