add aiofiles.threadpool.wrap to type stubs

Expose the singledispatch `wrap` function in aiofiles.threadpool stubs.
This was missing, causing mypy attr-defined errors when accessing
threadpool.wrap or calling wrap.register().

Closes #15624

Author: qorexdev

stubs/aiofiles/aiofiles/threadpool/__init__.pyi

@@ -9,7 +9,8 @@ from _typeshed import (
 from asyncio import AbstractEventLoop
 from collections.abc import Callable
 from concurrent.futures import Executor
-from typing import Literal, overload
+from functools import _SingleDispatchCallable
+from typing import Any, Literal, overload
 from typing_extensions import TypeAlias
 
 from ..base import AiofilesContextManager
@@ -98,6 +99,8 @@ def open(
     executor: Executor | None = None,
 ) -> AiofilesContextManager[_UnknownAsyncBinaryIO]: ...
 
+wrap: _SingleDispatchCallable[Any]
+
 stdin: AsyncTextIndirectIOWrapper
 stdout: AsyncTextIndirectIOWrapper
 stderr: AsyncTextIndirectIOWrapper