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

Start with the supplied Python reproducer and compare the dict comprehension with the working dict constructor and dict.fromkeys cases. Done means mypy accepts the comprehension assigning literal keys to Dict[Literal[1, 2, 3], None] without the incompatible key-expression error.