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

Issue にある Python の再現例から始め、Final[Literal] のケースを含め、mypy が Literal への引数をどのように検証しているかを追跡します。s を使用する例が受け入れられ、既存の無効な Literal のチェックが引き続きエラーを報告すれば完了です。関連する type-checker のテストでこれを検証してください。