Fix Python 3.9 compatibility

- Update requires-python to >=3.9 (was >=3.10)
- Regenerate poetry.lock to include Python 3.9 compatible versions
- Poetry will automatically select:
  - LangChain 0.3.x for Python 3.9
  - LangChain 1.x for Python 3.10+

Author: fede-kamel