Mark stub-only private symbols as @type_check_only in third-party stubs (#14545)

stubs/greenlet/greenlet/_greenlet.pyi

@@ -2,7 +2,7 @@ import sys
 from collections.abc import Callable
 from contextvars import Context
 from types import FrameType, TracebackType
-from typing import Any, Literal, Protocol, overload
+from typing import Any, Literal, Protocol, overload, type_check_only
 from typing_extensions import TypeAlias
 
 _TraceEvent: TypeAlias = Literal["switch", "throw"]
@@ -19,6 +19,7 @@ GREENLET_USE_TRACING: bool
 # to pass this around, can still pass it around without having to ignore type errors...
 _C_API: object
 
+@type_check_only
 class _ParentDescriptor(Protocol):
     def __get__(self, obj: greenlet, owner: type[greenlet] | None = None) -> greenlet | None: ...
     def __set__(self, obj: greenlet, value: greenlet) -> None: ...