Bugfix/signature replace and pydantic 2.10

[mikeedjones]

Pydantic 2.10 includes a change to model_fields turning it into a property. Dspy's Signature.replace method cannot replace that property. This PR updates the replacement to also target __pydantic_fields__.

• Reorganized imports in dspy/signatures/signature.py for better readability and maintainability.
• Simplified the OldSignature class definition by removing the unnecessary Signature base class.
• Updated requirements-dev.txt to include dependency datamodel-code-generator==0.26.3 and sorted reqs

[okhat]

Thank you so much @mikeedjones !