Fix mypy CI job

[AlexWaygood]

The 3.13 job seems to have started failing, not sure why: https://github.com/PyCQA/flake8-pyi/actions/runs/10469760712/job/28993477108?pr=497

[AlexWaygood]

I guess we could also just use a Python 3.12 installation for both mypy jobs and run with --target-version 3.13; no strong opinion on which is better there

[srittau]

I like using --target-version. The version mypy runs on shouldn't be important, and using a relatively new, but stable Python version seems to be the safest to me.

[AlexWaygood]

oh, we need to run the mypy-3.8 job using Python 3.8, or the conditional dependency ast_decompiler isn't installed, which is necessary for typechecking to succeed on Python 3.8

[srittau]

oh, we need to run the mypy-3.8 job using Python 3.8, or the conditional dependency ast_decompiler isn't installed, which is necessary for typechecking to succeed on Python 3.8

Could you add a comment about that to the yml file?

see comments

[AlexWaygood]

oh, we need to run the mypy-3.8 job using Python 3.8, or the conditional dependency ast_decompiler isn't installed, which is necessary for typechecking to succeed on Python 3.8

Could you add a comment about that to the yml file?

done!

[github-actions]

This change has no effect on typeshed. 🤖🎉