[red-knot] understand the Literal[] special form in annotations

[carljm]

No description provided.

[MichaReiser]

This is to teach red knot that mode is a union over w or r for def open(mode: Literal["w", "r"])

[carljm]

This is to teach red knot that mode is a union over w or r for def open(mode: Literal["w", "r"])

In order to fully handle this particular case, we will also have to implement #13693. But that issue isn't a blocker for this one. If we implement inference of Literal in type annotations (this issue) such that we understand this case which doesn't have the annotation on a function parameter:

from typing import Literal

x: Literal[10] = 10

def f():
    # currently we reveal `@Todo` here
    reveal_type(x)  # revealed: Literal[10]

then the parameter-annotation case @MichaReiser described will fall out naturally from #13693