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

Fixes #3984

stdlib/2/subprocess.pyi

@@ -98,7 +98,7 @@ class Popen(Generic[_T]):
                 startupinfo: Optional[Any] = ...,
                 creationflags: int = ...) -> Popen[bytes]: ...
 
-    def poll(self) -> int: ...
+    def poll(self) -> Optional[int]: ...
     def wait(self) -> int: ...
     # morally: -> Tuple[Optional[bytes], Optional[bytes]]
     def communicate(self, input: Optional[_TXT] = ...) -> Tuple[bytes, bytes]: ...