(🎁) Deprecate python 3.7 support

[KotlinIsland]

Python 3.7 is an old reference (deprecated), maybe mypy should drop support for it 😳

PEP 537 -- Python 3.7 Release Schedule

It's actually deprecated as of 2023-06, but close enough.

[JelleZijlstra]

3.7 has now reached EOL. Time to remove support?

[AlexWaygood]

As @ikonst has noted elsewhere, the tests on 3.7 are by far the slowest in CI, so it would definitely be a nice quality of life improvement to be able to ditch them :)

[JelleZijlstra]

Isn't some of that because we run a subset of tests on 3.7 only as it's the earliest supported version? Not sure that would be any better if we run them on 3.8 instead.

[AlexWaygood]

Isn't some of that because we run a subset of tests on 3.7 only as it's the earliest supported version?

I wasn't aware that we did that!

[JelleZijlstra]

If we don't drop support for 3.7 in the next release, we need to bump our typing-extensions requirements: #15543 (comment)

[ikonst]

I think the issue I filed (#15303) was about windows running tests nearly 2x slower. Don't think 3.7 had to do with it.

[hauntsaninja]

Let's pull the trigger and remove support for running under 3.7 (not checking). 3.7 is getting annoying. It's fallen to fewer than 10% of mypy users and I bet the fraction is even smaller for latest mypy version.