Langchain dependency update

```ModuleNotFoundError``` When working with langchain v1 in py/src/braintrust/wrappers/langchain.py
For example ```ModuleNotFoundError: No module named 'langchain.schema'```

This is due to langchain v1 moving these submodules to langchain_classic, see https://github.com/braintrustdata/braintrust-sdk-python/pull/102