Add @disjoint_base decorator to the third-party stubs (#14716)

stubs/greenlet/greenlet/_greenlet.pyi

@@ -3,7 +3,7 @@ from collections.abc import Callable
 from contextvars import Context
 from types import FrameType, TracebackType
 from typing import Any, Literal, Protocol, overload, type_check_only
-from typing_extensions import TypeAlias
+from typing_extensions import TypeAlias, disjoint_base
 
 _TraceEvent: TypeAlias = Literal["switch", "throw"]
 _TraceCallback: TypeAlias = Callable[[_TraceEvent, tuple[greenlet, greenlet]], object]
@@ -27,6 +27,7 @@ class _ParentDescriptor(Protocol):
 class GreenletExit(BaseException): ...
 class error(Exception): ...
 
+@disjoint_base
 class greenlet:
     @property
     def dead(self) -> bool: ...