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

stubs/boltons/boltons/setutils.pyi

@@ -1,10 +1,11 @@
 from collections.abc import Collection, Container, Generator, Iterable, Iterator, MutableSet
 from itertools import islice
-from typing import Any, Literal, Protocol, SupportsIndex, TypeVar, overload
+from typing import Any, Literal, Protocol, SupportsIndex, TypeVar, overload, type_check_only
 from typing_extensions import Self
 
 _T_co = TypeVar("_T_co", covariant=True)
 
+@type_check_only
 class _RSub(Iterable[_T_co], Protocol):
     def __new__(cls: type[_RSub[_T_co]], param: list[_T_co], /) -> _RSub[_T_co]: ...
 

stubs/boltons/boltons/typeutils.pyi

@@ -1,6 +1,7 @@
-from typing import Any, Literal, Protocol
+from typing import Any, Literal, Protocol, type_check_only
 from typing_extensions import Self
 
+@type_check_only
 class _Sentinel(Protocol):
     def __bool__(self) -> Literal[False]: ...
     def __copy__(self) -> Self: ...