Support multidict 5

[RhinosF1]

🐣 Is your feature request related to a problem? Please describe.

We want to keep what we install up to date but our checks show aiohttp blocks multidict 5 (See travis on FOSSBots/bots-pip-auto#113)

💡 Describe the solution you'd like

Allow multidict 5 with aiohttp
❓ Describe alternatives you've considered

N/A

📋 Additional context

[asvetlov]

Thanks for raising the question; I just forgot to relax the dependency.
Please let me propose an alternative PR which raises the top cap; it should be backward-compatible without any issue.

[RhinosF1]

@asvetlov: Multidict 5+ doesn't support python 3.5 which I think you do. That's why I chose what I did.

[RhinosF1]

Could we get a version on PyPi with this on @asvetlov so I can fix our install list?

[asvetlov]

The next aiohttp will not support Python 3.5 as well (python_requires='>=3.6')