Extend "in" type narrowing pattern to support literals

[erictraut]

This is related to this mypy issue.

from typing import Literal


def func(a: Literal[1, 2, 3]):
    x = (1, 2)
    assert a in x
    b: Literal[1, 2] = a

[erictraut]

This is addressed in pyright 1.1.203, which I just published. It will also be included in the next release of pylance.