Do not raise `unused-awaitable` when it's already `await`ed

[GideonBear]

Feature

Example:

from collections.abc import Generator
from typing import Any

from typing_extensions import Self


class MyAwaitable:
    def __await__(self) -> Generator[Any, None, Self]:
        return self.do_something().__await__()

    async def do_something(self) -> Self:
        # await self.do_async_stuff()
        return self


async def _() -> None:
    MyAwaitable()  # (correct) error: Value of type "MyAwaitable" must be used  [unused-awaitable]
    await MyAwaitable()  # (incorrect, already awaited) error: Value of type "MyAwaitable" must be used  [unused-awaitable]
    await (await MyAwaitable())  # (again incorrect) error: Value of type "MyAwaitable" must be used  [unused-awaitable]

Pitch

I've encountered this while creating a class with an async constructor using __await__, like this:

class AsyncClass:
    def __await__(self) -> Generator[Any, None, Self]:
        return self.__ainit__().__await__()

    async def __ainit__(self) -> Self:
        return self

I don't think it's very useful to suggest adding an await when it's already there.

This could be done by checking if the object that's an unused Awaitable is an await node.