Add msvcrt.locking() and associated constants (#3607)

stdlib/2and3/msvcrt.pyi

@@ -2,5 +2,13 @@
 
 # NOTE: These are incomplete!
 
+LK_LOCK: int
+LK_NBLCK: int
+LK_NBRLCK: int
+LK_RLCK: int
+LK_UNLCK: int
+
+def locking(fd: int, mode: int, nbytes: int) -> None: ...
+
 def get_osfhandle(file: int) -> int: ...
 def open_osfhandle(handle: int, flags: int) -> int: ...