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

stubs/dockerfile-parse/dockerfile_parse/parser.pyi

@@ -1,6 +1,6 @@
 import logging
 from collections.abc import Mapping, Sequence
-from typing import IO, ClassVar, TypedDict
+from typing import IO, ClassVar, TypedDict, type_check_only
 
 from .util import Context
 
@@ -19,6 +19,7 @@ class Labels(KeyValues): ...
 class Envs(KeyValues): ...
 class Args(KeyValues): ...
 
+@type_check_only
 class _InstructionDict(TypedDict):
     instruction: str
     startline: int