fix Type for Popen.communicate

Author: mr-c

typeshed/3/subprocess.pyi

@@ -1162,7 +1162,7 @@ class Popen(Generic[AnyStr]):
                     input: Optional[AnyStr] = ...,
                     timeout: Optional[float] = ...,
                     # morally this should be optional
-                    ) -> Tuple[AnyStr, AnyStr]: ...
+                    ) -> Tuple[Optional[AnyStr], AnyStr]: ...
     def send_signal(self, signal: int) -> None: ...
     def terminate(self) -> None: ...
     def kill(self) -> None: ...