Added stub for mutex(py2) (#1509)

stdlib/2/mutex.pyi

@@ -0,0 +1,15 @@
+# Source: https://hg.python.org/cpython/file/2.7/Lib/mutex.py
+
+from collections import deque
+from typing import Any, Callable, TypeVar
+
+_ArgType = TypeVar('_ArgType')
+
+class mutex:
+    locked = ...  # type: bool
+    queue = ...  # type: deque
+    def __init__(self) -> None: ...
+    def test(self) -> bool: ...
+    def testandset(self) -> bool: ...
+    def lock(self, function: Callable[[_ArgType], Any], argument: _ArgType) -> None: ...
+    def unlock(self) -> None: ...