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

stdlib/urllib/request.pyi

@@ -6,7 +6,7 @@ from email.message import Message
 from http.client import HTTPConnection, HTTPMessage, HTTPResponse
 from http.cookiejar import CookieJar
 from re import Pattern
-from typing import IO, Any, ClassVar, NoReturn, Protocol, TypeVar, overload
+from typing import IO, Any, ClassVar, NoReturn, Protocol, TypeVar, overload, type_check_only
 from typing_extensions import TypeAlias, deprecated
 from urllib.error import HTTPError as HTTPError
 from urllib.response import addclosehook, addinfourl
@@ -237,6 +237,7 @@ class ProxyDigestAuthHandler(BaseHandler, AbstractDigestAuthHandler):
     auth_header: ClassVar[str]  # undocumented
     def http_error_407(self, req: Request, fp: IO[bytes], code: int, msg: str, headers: HTTPMessage) -> _UrlopenRet | None: ...
 
+@type_check_only
 class _HTTPConnectionProtocol(Protocol):
     def __call__(
         self,