Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/@python2/mutex.pyi

@@ -0,0 +1,12 @@
+from typing import Any, Callable, Deque, TypeVar
+
+_T = TypeVar("_T")
+
+class mutex:
+    locked: bool
+    queue: Deque[Any]
+    def __init__(self) -> None: ...
+    def test(self) -> bool: ...
+    def testandset(self) -> bool: ...
+    def lock(self, function: Callable[[_T], Any], argument: _T) -> None: ...
+    def unlock(self) -> None: ...