Let "Literal" accept another Literal type variable

Feature

Currently Literal doesn't accept another Literal or Final[Literal]. For example:

from typing_extensions import Final, Literal

s: Final[Literal['ccc']] = 'ccc'
s1: Literal[s] = s

outputs:

a.py:4: error: Parameter 1 of Literal[...] is invalid
a.py:4: error: Variable "a.s" is not valid as a type
a.py:4: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
Found 2 errors in 1 file (checked 1 source file)

Pitch

This is particularly useful when there are some defined constants and people refer to those constants rather than the actual value. For example, some of these socket constants: https://docs.python.org/3/library/socket.html#socket.BDADDR_ANY

Inizia con la riproduzione in Python nell'issue e segui come mypy valida gli argomenti di Literal, incluso il caso Final[Literal]. Il lavoro è completato quando l'esempio che usa s viene accettato e i controlli esistenti per i Literal non validi continuano a segnalare errori; verificalo con i test pertinenti del type-checker.