Fix handling of optionals in mypy plugin

[dmontagu]

I think this may resolve various occurrences of mypy issues with optional, such as #9007

Mirrors the changes to the dataclasses plugin in python/mypy#15571

Fix #9007
Fix #8760

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[codspeed-hq]

CodSpeed Performance Report

Merging #9008 will not alter performance

_{Comparing dmontagu/fix-mypy-optional-stuff (2ce76e8) with main (18d39fe)}

Summary

✅ 10 untouched benchmarks

[davidhewitt]

Is there a test case we should add here?

Also, I wonder, should we be setting a minimum supported version of mypy for the plugin? I don't know if it's practical for us to throw an exception if the mypy version is too old?

[Viicos]

Also, I wonder, should we be setting a minimum supported version of mypy for the plugin? I don't know if it's practical for us to throw an exception if the mypy version is too old?

I think it would make sense to drop some older versions of mypy, especially because before around 1.1 support for dataclass_transform wasn't really great, and I had some issues when tweaking the plugin with this

[davidhewitt]

1.1.1 is a year old, so that might be a suitable cutoff point. Or we could try to go further, 1.5.1 is 6 months old. It would be nice with either limit if we could fail gracefully if users try an older mypy.

[sydney-runkle]

Nice, thanks @dmontagu

[dmontagu]

I have added a test, but I'll note that the only affected version that we test against is mypy 1.5.1 (and we install 1.1.1 by default in pydantic). We should probably update the versions we test against soon. I'm not prepared to do that in this PR though.

[sydney-runkle]

Just marking as 'awaiting author revision` until the tests are passing 👍

[sydney-runkle]

Great, thank you very much @dmontagu !!