Add __all__ (#13704)

stubs/singledispatch/@tests/stubtest_allowlist.txt

@@ -1,2 +0,0 @@
-# TODO: missing from stub
-singledispatch.__all__

stubs/singledispatch/singledispatch.pyi

@@ -29,3 +29,5 @@ class singledispatchmethod(Generic[_T]):
     @overload
     def register(self, cls: type[Any], method: Callable[..., _T]) -> Callable[..., _T]: ...
     def __get__(self, obj: _S, cls: type[_S] | None = ...) -> Callable[..., _T]: ...
+
+__all__ = ["singledispatch", "singledispatchmethod"]