Wrap weakref.ref instead of subclassing to fix cloudpickle serialization

[edoakes]

Change Summary

See context in #6763.

The existing _PydanticWeakref cannot be serialized due to a known issue serializing weakref.ref objects.

This change works around the issue by updating _PydanticWeakref to be a wrapper instead of a subclass.

Along with pydantic/pydantic-core#1006, this should enable pydantic model definitions to be cloudpickled.

Related issue number

#6763

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

[edoakes]

@davidhewitt PTAL

[davidhewitt]

LGTM, thanks!

For reference it looks like the original discussion about why _PydanticWeakRef exists and alternatives can be found in #6681 (comment)