Upgrade aiohttp to 0.21
Upgrade aiohttp to 0.21

We're treating ClientSession.close as a coroutine, which was a change
made in 0.21, so increase the requirement to match.

andreasots/lrrbot,mrphlip/lrrbot,mrphlip/lrrbot,mrphlip/lrrbot,andreasots/lrrbot,andreasots/lrrbot