Deprecation warning

[eMerzh]

Hi, i'm running the version 2.0 with aiohttp = "==3.6.2"
but i got a bunch of

site-packages/aiohttp_sse/init.py:59: DeprecationWarning: loop property is deprecated
self._loop = request.app.loop

i thing we should get_running_loop instead or smth like that...

Thanks a lot for the lib :)

[Sam Bull]

#428