Fix `AsyncResponse.iter_lines()`

[vrslev]

Doesn't work now. Causes an exception:TypeError: 'async for' requires an object with __aiter__ method, got coroutine. MRE:

import asyncio

import niquests

async def main() -> None:
    async with niquests.AsyncSession() as session:
        response: niquests.AsyncResponse = session.post(..., stream=True)
        async for line in response.iter_lines():
            print(line)

asyncio.run(main())