Speed up mypy regr test in CI

[JelleZijlstra]

No description provided.

[AlexWaygood]

Woah, that used to take 5-6 minutes -- how is it taking ~19 minutes on recent CI runs?? What on earth changed to cause that much of a regression?

[AlexWaygood]

I think it's because 29bffca means that we're now using Python 3.15 to run regr_test.py, which means we're installing an uncompiled version of mypy in CI.

[AlexWaygood]

It shouldn't be necessary to actually use Python 3.15 to run the script -- we should be able to run mypy with --python-version=3.15 inside the script without mypy being actually run with 3.15.

[JelleZijlstra]

Ah good point. Yeah this didn't actually help much, it seemed to help locally.

[JelleZijlstra]

Switched it back to 3.14 and now it takes 5 min again. Kept a bugfix Codex found where we were missing environment setup failures because we weren't awaiting some futures.

[AlexWaygood]

we weren't awaiting some futures

Hum, but these aren't async futures, so I don't think they need to be awaited -- what was codex's full reasoning here, OOI?

[hauntsaninja]

The exception will never propagate if you don't .result() it