We are rely on __del__ in the library a lot.
For graceful finalization for bare aiohttp.request() and family after merging #536 we should do session detach in https://github.com/KeepSafe/aiohttp/pull/536/files#diff-7dd84b5ef8d5eea2de1dfc5329411dfcR472

It doesn't work on Python 3.3. I can add weakref tricks, sure. But prefer dropping Python 3.3 at all.

Thoughts?