[urllib.requests] Allow install_opener to be reset (#15477)

Author: ewerybody

stdlib/urllib/request.pyi

@@ -92,7 +92,7 @@ else:
         context: None = None,
     ) -> _UrlopenRet: ...
 
-def install_opener(opener: OpenerDirector) -> None: ...
+def install_opener(opener: OpenerDirector | None) -> None: ...
 def build_opener(*handlers: BaseHandler | Callable[[], BaseHandler]) -> OpenerDirector: ...
 
 if sys.version_info >= (3, 14):