Add missing metaclasses in ast and contextlib (#8497)

stdlib/ast.pyi

@@ -5,21 +5,25 @@ from typing import Any, TypeVar, overload
 from typing_extensions import Literal
 
 if sys.version_info >= (3, 8):
-    class Num(Constant):
+    class _ABC(type):
+        if sys.version_info >= (3, 9):
+            def __init__(cls, *args: object) -> None: ...
+
+    class Num(Constant, metaclass=_ABC):
         value: complex
 
-    class Str(Constant):
+    class Str(Constant, metaclass=_ABC):
         value: str
         # Aliases for value, for backwards compatibility
         s: str
 
-    class Bytes(Constant):
+    class Bytes(Constant, metaclass=_ABC):
         value: bytes
         # Aliases for value, for backwards compatibility
         s: bytes
 
-    class NameConstant(Constant): ...
-    class Ellipsis(Constant): ...
+    class NameConstant(Constant, metaclass=_ABC): ...
+    class Ellipsis(Constant, metaclass=_ABC): ...
 
 if sys.version_info >= (3, 9):
     class slice(AST): ...

stdlib/contextlib.pyi

@@ -1,3 +1,4 @@
+import abc
 import sys
 from _typeshed import Self, StrOrBytesPath
 from abc import abstractmethod
@@ -133,7 +134,9 @@ class _RedirectStream(AbstractContextManager[_T_io]):
 class redirect_stdout(_RedirectStream[_T_io]): ...
 class redirect_stderr(_RedirectStream[_T_io]): ...
 
-class ExitStack:
+# In reality this is a subclass of `AbstractContextManager`;
+# see #7961 for why we don't do that in the stub
+class ExitStack(metaclass=abc.ABCMeta):
     def __init__(self) -> None: ...
     def enter_context(self, cm: AbstractContextManager[_T]) -> _T: ...
     def push(self, exit: _CM_EF) -> _CM_EF: ...
@@ -148,7 +151,9 @@ class ExitStack:
 _ExitCoroFunc: TypeAlias = Callable[[type[BaseException] | None, BaseException | None, TracebackType | None], Awaitable[bool]]
 _ACM_EF = TypeVar("_ACM_EF", bound=AbstractAsyncContextManager[Any] | _ExitCoroFunc)
 
-class AsyncExitStack:
+# In reality this is a subclass of `AbstractAsyncContextManager`;
+# see #7961 for why we don't do that in the stub
+class AsyncExitStack(metaclass=abc.ABCMeta):
     def __init__(self) -> None: ...
     def enter_context(self, cm: AbstractContextManager[_T]) -> _T: ...
     async def enter_async_context(self, cm: AbstractAsyncContextManager[_T]) -> _T: ...