Remove all 3.7 support

[kevinkjt2000]

Fixes #388

[kevinkjt2000]

While working on this, I realized that the tox.ini installs too many packages in py{38,39,310} environments. I don't know how to select groups with the tox-poetry plugin that we are using or if that is even supported. Those test environments should not have linter packages installed to try to minimize stuff like accidental import of typing-extensions at the top-level (yeah, I did this locally but gave it some thought before pushing that commit).

[ItsDrike]

I'm pretty sure tox-poetry doesn't support poetry groups, as it was last updated in august 2021, at which point there weren't even a thing yet. That is, unless it somehow supports passing args to poetry directly, but I haven't seen that anywhere in their docs. Unless we handle things differently somehow, I don't think we can get rid of installing the lint dependencies in tox envs.

It might be a good idea to open up an issue about this.

[PerchunPak]

I found a workaround for Python 3.6 in tests, maybe remove it here?