… the right params

See https://github.com/python/cpython/pull/130309/changes#r2663516538.

These functions don't annotate parameters for values used in single-dispatching (`item`), they annotate parameters one later (`arg`).

This being legal relies on a bug -- pythonGH-84644, which is that `singledispatch` doesn't verify the annotation is on the "first" parameter.
I think these functions should look like

```diff
            @functools.singledispatchmethod
-           def func(self, item, arg: int) -> str:
+           def func(self, item: int, arg) -> str:
                return str(item)
            @func.register
-           def _(self, item, arg: bytes) -> str:
+           def _(self, item: bytes, arg) -> str:
                return str(item)
```

(and signature tests updated accordingly)

In practice, it doesn't matter how you annotate `func` here, because it's a default callback. But what matters is that any function passed to `register()` annotates the target parameter (always first positional in `singledispatch` and always second positional in `singledispatchmethod` (unless staticmethod, then the first) etc.) for the value to single-dispatch on; not some other, unrelated parameter (or return type, as presented in pythonGH-84644).

Let's have a class with the same registree signature as these from the test:

```py
class A:
    @functools.singledispatchmethod
    def func(self, item, arg: int) -> str:
        return 'argint'
    @func.register
    def _(self, item, arg: bytes) -> str:
        return 'argbytes'

a = A()
print(a.func(b'', 1))
```

For this code, the output would be `argbytes`, even though `arg` is an integer `1`.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>