Change return annotation of Popen.poll to Optional[int] (#3986)

Fixes #3984

stdlib/3/subprocess.pyi

@@ -1155,7 +1155,7 @@ class Popen(Generic[AnyStr]):
                     start_new_session: bool = ...,
                     pass_fds: Any = ...) -> Popen[Any]: ...
 
-    def poll(self) -> int: ...
+    def poll(self) -> Optional[int]: ...
     if sys.version_info >= (3, 7):
         def wait(self, timeout: Optional[float] = ...) -> int: ...
     else: