[enum] Add Flag._numeric_repr_ (#15061)

Author: guoci

stdlib/enum.pyi

@@ -255,6 +255,7 @@ _auto_null: Any
 class Flag(Enum):
     _name_: str | None  # type: ignore[assignment]
     _value_: int
+    _numeric_repr_: Callable[[int], str]
     @_magic_enum_attr
     def name(self) -> str | None: ...  # type: ignore[override]
     @_magic_enum_attr