new union syntax doesn't work on `Literal`s

```py
from __future__ import annotations

from typing import Literal

Foo = Literal[1, 2]

Bar = Foo | Literal[3] # error: Unsupported left operand type for | ("object")
```
https://mypy-play.net/?mypy=latest&python=3.10&gist=3a429dfa3e9e3b692aac25887027ab69