Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/atexit.pyi

@@ -0,0 +1,7 @@
+from typing import Any, Callable
+
+def _clear() -> None: ...
+def _ncallbacks() -> int: ...
+def _run_exitfuncs() -> None: ...
+def register(func: Callable[..., Any], *args: Any, **kwargs: Any) -> Callable[..., Any]: ...
+def unregister(func: Callable[..., Any]) -> None: ...