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

Start by running the provided main.py repro with mypy and compare the reported diagnostics for getattr and getattribute. Trace the type-checking entry points for these special methods; done means the repro produces “Success: no issues found in 1 source file” without weakening ordinary object.getattribute checks.