multiprocessing.pool: Use Literal and add __all__ (#6815)

Author: sobolevn

stdlib/multiprocessing/pool.pyi

@@ -2,10 +2,13 @@ import sys
 from _typeshed import Self
 from contextlib import AbstractContextManager
 from typing import Any, Callable, Generic, Iterable, Iterator, Mapping, TypeVar
+from typing_extensions import Literal
 
 if sys.version_info >= (3, 9):
     from types import GenericAlias
 
+__all__ = ["Pool", "ThreadPool"]
+
 _PT = TypeVar("_PT", bound=Pool)
 _S = TypeVar("_S")
 _T = TypeVar("_T")
@@ -115,11 +118,11 @@ class ThreadPool(Pool, AbstractContextManager[ThreadPool]):
 
 # undocumented
 if sys.version_info >= (3, 8):
-    INIT: str
-    RUN: str
-    CLOSE: str
-    TERMINATE: str
+    INIT: Literal["INIT"]
+    RUN: Literal["RUN"]
+    CLOSE: Literal["CLOSE"]
+    TERMINATE: Literal["TERMINATE"]
 else:
-    RUN: int
-    CLOSE: int
-    TERMINATE: int
+    RUN: Literal[0]
+    CLOSE: Literal[1]
+    TERMINATE: Literal[2]