Fix PyPI upload failure by removing git dependency

[tomdurrant]

Summary

Removes the notebooks optional dependency that was causing PyPI upload failures.

Problem

The v0.6.3 release failed to upload to PyPI with this error:

ERROR: Can't have direct dependency: rompy-notebooks @ 
git+https://github.com/rom-py/rompy-notebooks.git@package ; extra == "notebooks"

PyPI does not allow direct git dependencies in package metadata, even in optional extras, because they are not reproducible or verifiable according to PEP 440.

Solution

Removed the notebooks optional dependency from pyproject.toml. This dependency is not needed for package distribution because:

1. The docs build workflow (.github/workflows/build-docs.yml) already clones rompy-notebooks directly from GitHub
2. The notebooks are copied into the docs directory during the build process
3. The package is installed with pip install .[docs], not [notebooks]

Testing

• The change allows the package to be built and uploaded to PyPI without validation errors
• Docs build workflow remains unaffected as it handles notebooks separately

Fixes the PyPI upload failure in release v0.6.3.