Fix async prefix for groupby type annotation overloads. (#64)

The overload functions are not generator functions, so mypy currently infers
that they return an awaitable that yields an AsyncIterator rather than
that they return an AsyncIterator directly.

Author: bwhmather

asyncstdlib/itertools.py

@@ -512,21 +512,21 @@ async def identity(x: T) -> T:
 
 
 @overload  # noqa: F811
-async def groupby(  # noqa: F811
+def groupby(  # noqa: F811
     iterable: AnyIterable[T],
 ) -> AsyncIterator[Tuple[T, AsyncIterator[T]]]:
     ...
 
 
 @overload  # noqa: F811
-async def groupby(  # noqa: F811
+def groupby(  # noqa: F811
     iterable: AnyIterable[T], key: None
 ) -> AsyncIterator[Tuple[T, AsyncIterator[T]]]:
     ...
 
 
 @overload  # noqa: F811
-async def groupby(  # noqa: F811
+def groupby(  # noqa: F811
     iterable: AnyIterable[T], key: Union[Callable[[T], R], Callable[[T], Awaitable[R]]]
 ) -> AsyncIterator[Tuple[R, AsyncIterator[T]]]:
     ...