stdlib: fix many attributes which are read-only at runtime but read-write in the stub (#7395)

stdlib/multiprocessing/dummy/__init__.pyi

@@ -3,6 +3,7 @@ import threading
 import weakref
 from queue import Queue as Queue
 from typing import Any, Callable, Iterable, Mapping, Sequence
+from typing_extensions import Literal
 
 from .connection import Pipe as Pipe
 
@@ -39,7 +40,8 @@ class DummyProcess(threading.Thread):
     _parent: threading.Thread
     _pid: None
     _start_called: int
-    exitcode: int | None
+    @property
+    def exitcode(self) -> Literal[0] | None: ...
     def __init__(
         self,
         group: Any = ...,