Support using Literal on __getattr__ and __getattribute__

Hi! Feature request for using Literal on __getattr__ and __getattribute__.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Literal

LiteralStrHello = Literal["hello"]
LiteralStrWorld = Literal["world"]


class A:
    def __getattr__(self, name: LiteralStrHello) -> LiteralStrWorld:
        assert name == "hello"
        return "world"

    def __getattribute__(self, name: LiteralStrHello) -> LiteralStrWorld:
        assert name == "hello"
        return "world"

• What is the actual behavior/output?

main.py:8: error: Invalid signature "def (main.A, Literal['hello']) -> Literal['world']" for "__getattr__"
main.py:12: error: Invalid signature "def (main.A, Literal['hello']) -> Literal['world']" for "__getattribute__"
main.py:12: error: Argument 1 of "__getattribute__" is incompatible with supertype "object"; supertype defines the argument type as "str"
Found 3 errors in 1 file (checked 1 source file)

• What is the behavior/output you expect?

Success: no issues found in 1 source file

• What are the versions of mypy and Python you are using? Mypy:mypy 0.770+dev.5336f272c2ac625bd40a79ab3dbbe3d548f147b8, Python:3.8.0
  Do you see the same issue after installing mypy from Git master? yes

まず、提供されている main.py の再現コードを mypy で実行し、getattr と getattribute について報告される診断を比較します。これらの特殊メソッドに対する型チェックのエントリーポイントを追跡します。再現コードが、通常の object.getattribute のチェックを弱めることなく “Success: no issues found in 1 source file” を出力すれば完了です。