Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/asyncio/threads.pyi

@@ -0,0 +1,7 @@
+import sys
+from typing import Any, Callable, TypeVar
+
+_T = TypeVar("_T")
+
+if sys.version_info >= (3, 9):
+    async def to_thread(__func: Callable[..., _T], *args: Any, **kwargs: Any) -> _T: ...