asyncio.subprocess: returncode is Optional (#3981)

Author: kkty

stdlib/3/asyncio/subprocess.pyi

@@ -30,7 +30,7 @@ class Process:
                  protocol: protocols.BaseProtocol,
                  loop: events.AbstractEventLoop) -> None: ...
     @property
-    def returncode(self) -> int: ...
+    def returncode(self) -> Optional[int]: ...
     async def wait(self) -> int: ...
     def send_signal(self, signal: int) -> None: ...
     def terminate(self) -> None: ...