Summary

enable serialization of _thread._ThreadHandle instances.

Checklist

Documentation and Tests

• Added relevant tests that run with python tests/__main__.py, and pass.
• Added relevant documentation that builds in sphinx without error.
• Artifacts produced with the main branch work as expected under this PR.

Release Management

• Added "Fixes #NNN" in the PR body, referencing the issue (#NNN) it closes.
• Added a comment to issue #NNN, linking back to this PR.