Make sure that client response life cycle is shorter than client session

[asvetlov]

I observed a code like this on stack overflow:

async def fetch(url, data):
    async with ClientSession() as session:
        response = await session.post(url, data=data)

    return response

The function is really wrong (while it may work for short response body).
The problem is: a session is destroyed before response destruction.
If the response did not come as short data and IO is needed for await response.read() and family the code fails with a very cryptic exception.

ClientResponse should have a weak reference to ClientSession. When the session is gone weakref callback should raise an exception.