aiohttp 2.0 compatibility

[Nikolay Novik]

We need to fix compatibility with new aiohttp, library has request/response refactored and using nodejs parser

[Nicolas Delaby]

👍 let's start by running the tests against it.

[Vladimir Rutsky]

In aiohttp-cors we have specific configuration that runs on master branch of aiohttp:
https://github.com/aio-libs/aiohttp-cors/blob/master/.travis.yml#L39

Perhaphs we should add tox environment in aiohttp-sse for that too?