Dict comprehension from Iterable of Literal

Related to #7533
mypy 0.740, python 3.7.4, typing_extensions 3.7.4

dict comprehension won't recognize an Iterable of Literal as input.
Unlike in #7533, this is not limited to just tuples.

The dict constructor and dict.fromkeys have no problem.

from typing import Dict
from typing_extensions import Literal

T = Literal[1, 2, 3]
d: Dict[T, None]

d = {x: None for x in [1, 2, 3]}
# error: Key expression in dictionary comprehension has incompatible type "int"; expected type "Union[Literal[1], Literal[2], Literal[3]]"

d = dict(zip([1, 2, 3], [None, None, None]))  # no error
d = dict.fromkeys([1, 2, 3])  # no error

提供された Python 再現コードから始め、dict comprehension と、動作する dict constructor および dict.fromkeys のケースを比較します。mypy がリテラルキーを Dict[Literal[1, 2, 3], None] に割り当てる comprehension を、互換性のないキー式エラーなしで受け入れれば完了です。