Add/verify compatibility with `aiohttp` >=4.0

[marcinsulikowski]

aiohttp 4.0.x pre-releases are available (see: https://pypi.org/project/aiohttp/#history) but aioresponses currently requires aiohttp<4.0.0,>=2.0.0 making it impossible to use the two together. Support for the 4.x release should be added. Backward-incompatible changes are described here: https://docs.aiohttp.org/en/latest/changes.html#deprecations-and-removals.