[psutil]: Add missing __dir__ method to psutil.Popen (#14207)

stubs/psutil/psutil/__init__.pyi

@@ -232,6 +232,7 @@ class Popen(Process):
     def __enter__(self) -> Self: ...
     def __exit__(self, *args: object, **kwargs: object) -> None: ...
     def __getattribute__(self, name: str) -> Any: ...
+    def __dir__(self) -> list[str]: ...
 
 def pids() -> list[int]: ...
 def pid_exists(pid: int) -> bool: ...