[BUG] Missing PIL dependency

[pjavanrood]

Describe the bug

The Pillow (PIL) package is missing from the dependencies in pyproject.toml, causing import errors when running tasks that require image processing, such as mathvista(I think introduced in #1118 )

To Reproduce

1. Install lighteval from source: pip install -e .
2. Run an evaluation that uses image processing tasks (e.g., mathvista)
3. The evaluation will fail with an import error: ModuleNotFoundError: No module named 'PIL'

Expected behavior

The Pillow package should be included in the dependencies so that image processing tasks can run without requiring users to manually install it.

Version info

• Operating system: Any
• Lighteval version: 0.13.1.dev0 (or main branch)
• Python version: >=3.10
• The issue occurs when installing from source or when dependencies are not explicitly installed separately