I got the same issue. I am on python 3.6 and pip version 20.0.2. I also tried to separately install aiohttp
pip3 install aiohttp works fine. I think the dependency is false, does aiohttp 3.5 even exist?. I tried to install rasa for the last 2 h now trying different combinations of rasa versions and python versions, but none worked. Before aiohttp it was failing at tensorflow. I suggest to clone the code from github.