$ cat test1.py
async def coro():
    return

def aio() -> ():
    return coro()
$ mypy test1.py
test1.py:4: error: Syntax error in type annotation
test1.py:4: note: Suggestion: Use Tuple[T1, ..., Tn] instead of (T1, ..., Tn)
$ cat test2.py
import typing

async def coro():
    return

def aio() -> typing.Coroutine:
    return coro()
$ mypy test2.py
$