Fixes for Pydantic v2 compat

[kabirkhan]

Description

We have a mixture of imports for the pydantic BaseModel from both v2 and v1.
This PR standardizes imports around v1 by adding BaseModel, ValidationError and validator to the compat module. Fixes the need for cls.construct for the relations module.

Corresponding documentation PR

Types of change

Enhancment

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran all tests in tests and usage_examples/tests, and all new and existing tests passed. This includes
  • all external tests (i. e. pytest ran with --external)
  • all tests requiring a GPU (i. e. pytest ran with --gpu)
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[svlandeg]

Thanks, looks good!