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

Comienza con la reproducción en Python del issue y sigue cómo mypy valida los argumentos de Literal, incluido el caso de Final[Literal]. Se considera completado cuando el ejemplo que usa s es aceptado y las comprobaciones existentes de Literal no válidos siguen informando de errores; verifícalo con las pruebas relevantes del type-checker.