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

Inizia con il riproduttore Python fornito e confronta la dict comprehension con i casi funzionanti dict constructor e dict.fromkeys. Il lavoro è completato quando mypy accetta la comprehension che assegna chiavi letterali a Dict[Literal[1, 2, 3], None] senza l'errore di espressione della chiave incompatibile.