Fix timeout parameter to Popen.wait() -- it is optional (in both senses).

stdlib/3/subprocess.pyi

@@ -229,7 +229,7 @@ class Popen:
     def poll(self) -> int: ...
     if sys.version_info >= (3, 3):
         # 3.3 added timeout
-        def wait(self, timeout: float) -> int: ...
+        def wait(self, timeout: Optional[float] = ...) -> int: ...
     else:
         def wait(self) ->int: ...
     # Return str/bytes