TypedDict doesn't support Union[Literal] as key "string literal"

Possibly bug.
Related: https://github.com/python/mypy/issues/7644

Reproducible example

from typing_extensions import Literal, TypedDict

class TDict(TypedDict):
    KEY1: int
    KEY2: int

d1: TDict = {'KEY1': 1, 'KEY2': 2}

key2: Literal['KEY1']
d2: TDict = {key2: 1, 'KEY2': 2}

key3: Literal['KEY1', 'KEY2']
d3: TDict = {key3: 1, 'KEY2': 2}  # error: Expected TypedDict key to be string literal

I use mypy 0.740 and python3.6

首先使用 mypy 0.740 运行提供的 TypedDict 和 Literal 示例，重点关注报告“Expected TypedDict key to be string literal.”的 d3 赋值。当可复现的 union-of-literals 键案例能够与单个 literal 案例一致地被接受时，即表示完成。