Sandbox Improvements

[cretz]

What was changed

• Fix issue with isinstance and issubclass in sandbox
  • Made them be thread-local during sandbox run and if in sandbox, make sure to unwrap their parameters before running
• Properly resolve relative imports to their full names for checking against passthrough/restrictions
• Add with workflow.unsafe.imports_passed_through(): context manager for easier passthrough
• Change SandboxRestrictions.passthrough_modules to be a simple set of strings instead of the unnecessary full sandbox matcher used by other restrictions (💥 technically a breaking change in a marked-unstable API area)
• Added SandboxRestrictions.with_passthrough_modules helper to add passthrough modules to the immutable restriction set
• Did hack to make subclasses of restricted/proxy types at lease load. They still don't work if used though.
• Mark Pydantic datetime issue as a known issue
• Clarified passthrough module customization in README

Checklist

1. Closes [Bug] Fix README to use workflow_runner instead of runner in sandbox examples #203
2. Closes [Feature Request] Make it easier to pass through third party modules #204
3. Relates to [Bug] SandboxWorkflowRunner doesn't use correct Pydantic field types in some cases #207
4. Closes [Bug] Sandbox importer not respecting relative levels for pass through #208
5. Closes [Bug] Sandboxed classes should be able to extend restricted classes #210
6. Closes [Bug] Importing gevent causes sandbox errors #216