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

Start with the Python reproduction in the issue and trace how mypy validates arguments to Literal, including the Final[Literal] case. Done means the example using s is accepted and the existing invalid-Literal checks continue to report errors; verify this with the relevant type-checker tests.