Use extra for asyncio dependencies

[merwok]

Hello! The latest release for this library pulls in aiohttp and its dependencies unconditionally, which adds non-trivial burden to projects that don’t need it. Would you consider using a packaging extra so that people can opt-in?

[busunkim96]

That sounds alright to me. @crwilcox?

[crwilcox]

I think that's reasonable. Maybe we will want to use the same approach for there other asyncio/aiohttp packages

[merwok]

Nice, thanks!