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

stubs/pika/pika/credentials.pyi

@@ -1,8 +1,9 @@
 from logging import Logger
-from typing import ClassVar, Protocol
+from typing import ClassVar, Protocol, type_check_only
 
 from .spec import Connection
 
+@type_check_only
 class _Credentials(Protocol):
     TYPE: ClassVar[str]
     erase_on_connect: bool