[http.cookies] Fix wrong return type in BaseCookie.value_encode (#15267)

Author: b9788213

stdlib/http/cookies.pyi

@@ -47,7 +47,7 @@ class Morsel(dict[str, Any], Generic[_T]):
 class BaseCookie(dict[str, Morsel[_T]], Generic[_T]):
     def __init__(self, input: _DataType | None = None) -> None: ...
     def value_decode(self, val: str) -> tuple[_T, str]: ...
-    def value_encode(self, val: _T) -> tuple[_T, str]: ...
+    def value_encode(self, val: _T) -> tuple[str, str]: ...
     def output(self, attrs: list[str] | None = None, header: str = "Set-Cookie:", sep: str = "\r\n") -> str: ...
     __str__ = output
     def js_output(self, attrs: list[str] | None = None) -> str: ...