Drop support for python 3.7

[JWCook]

Python 3.7 is EOL, and it would make sense to drop support for it in the next minor release. That would allow several 3.7-specific workarounds to be removed.

If that would cause problems for anyone, please let me know.

[alessio-locatelli]

I can do this in 1-2 weeks (because this issue is not urgent) unless you want to handle this by yourself.

aiohttp dropped 3.7 in aio-libs/aiohttp#7336 and released https://github.com/aio-libs/aiohttp/releases/tag/v3.9.0 two days ago.

By the way, it will be useful to add https://github.com/asottile/pyupgrade that auto-updates syntax to a specified Python version.

[JWCook]

I started removing some workarounds for 3.7 in the integration tests (mostly just the ones that depend on AsyncMock). I'll submit what I have so far, and if you'd like you can add pyupgrade and make any other needed changes whenever you have the time? And yeah, it's definitely not urgent.