Add @type_check_only to stub-only private classes in stdlib (#14512)

stdlib/xml/etree/ElementInclude.pyi

@@ -1,7 +1,8 @@
 from _typeshed import FileDescriptorOrPath
-from typing import Final, Literal, Protocol, overload
+from typing import Final, Literal, Protocol, overload, type_check_only
 from xml.etree.ElementTree import Element
 
+@type_check_only
 class _Loader(Protocol):
     @overload
     def __call__(self, href: FileDescriptorOrPath, parse: Literal["xml"], encoding: str | None = None) -> Element: ...

stdlib/xml/etree/ElementTree.pyi

@@ -335,6 +335,7 @@ class C14NWriterTarget:
 # The target type is tricky, because the implementation doesn't
 # require any particular attribute to be present. This documents the attributes
 # that can be present, but uncommenting any of them would require them.
+@type_check_only
 class _Target(Protocol):
     # start: Callable[str, dict[str, str], Any] | None
     # end: Callable[[str], Any] | None