Use union_schema with Type[Union[...]]

[JeanArhancet]

Change Summary

Use union_schema and not is_subclass_schema if it's Type[Union[...]]

Related issue number

Resolves #6815

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[JeanArhancet]

please review

[adriangb]

I'm not sure that isinstance vs issubclass is the issue (although if it is flipped that would be a bug so this PR may make sense anyway).

The bigger issue IMO is that isinstance and issubclass are not meant to work with parametrized generics and unions. If you do issubclass(int, int|str) you get an error and isinstance(int, int|str) is False.

I think the best we can do (and what I'm guessing V1 did) is check if the thing inside of type[...] is a union and if so convert it to a union of types.

[JeanArhancet]

@adriangb I changed the implementation WDYT?

[adriangb]

Looks good to me. While it is not a universal solution this is a relatively common use case and handling it is just a couple lines of code, so it seems worth it to me.