Improves Popen types in psutil (#6074)

stubs/psutil/psutil/__init__.pyi

@@ -1,5 +1,5 @@
 import sys
-from typing import Any, Callable, ContextManager, Iterable, Iterator
+from typing import Any, Callable, ContextManager, Iterable, Iterator, TypeVar
 
 from ._common import (
     AIX as AIX,
@@ -160,13 +160,13 @@ class Process:
     def kill(self) -> None: ...
     def wait(self, timeout: int | None = ...) -> int: ...
 
+_Popen = TypeVar("_Popen", bound="Popen")
+
 class Popen(Process):
     def __init__(self, *args, **kwargs) -> None: ...
-    def __dir__(self): ...
-    def __enter__(self): ...
-    def __exit__(self, *args, **kwargs): ...
-    def __getattribute__(self, name): ...
-    def wait(self, timeout: Any | None = ...): ...
+    def __enter__(self: _Popen) -> _Popen: ...
+    def __exit__(self, *args, **kwargs) -> None: ...
+    def __getattribute__(self, name: str) -> Any: ...
 
 def pids() -> list[int]: ...
 def pid_exists(pid: int) -> bool: ...