Add default value to BaseContext.Pipe() (#3479)

stdlib/3/multiprocessing/context.pyi

@@ -38,7 +38,7 @@ class BaseContext(object):
     # TODO: change return to SyncManager once a stub exists in multiprocessing.managers
     def Manager(self) -> Any: ...
     # TODO: change return to Pipe once a stub exists in multiprocessing.connection
-    def Pipe(self, duplex: bool) -> Any: ...
+    def Pipe(self, duplex: bool = ...) -> Any: ...
 
     def Barrier(self,
                 parties: int,