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

stubs/fpdf2/fpdf/encryption.pyi

@@ -1,7 +1,7 @@
 from _typeshed import Incomplete, SupportsLenAndGetItem
 from collections.abc import Generator, Iterable
 from logging import Logger
-from typing import ClassVar, Protocol, TypeVar, overload
+from typing import ClassVar, Protocol, TypeVar, overload, type_check_only
 from typing_extensions import TypeAlias
 
 from .enums import AccessPermission, EncryptionMethod
@@ -15,6 +15,7 @@ LOGGER: Logger
 
 import_error: ImportError | None
 
+@type_check_only
 class _SupportsGetItem(Protocol[_T_co]):
     def __getitem__(self, k: int, /) -> _T_co: ...