fix: add missing scipy dependency for bitsandbytes

[glennko]

Summary

• Add scipy >= 1.0.0 to dependencies to fix bitsandbytes import errors
• Resolves ModuleNotFoundError: No module named 'scipy' when importing xturing modules
• Enables test suite to run without import failures

Problem

bitsandbytes==0.41.1 uses scipy.stats.norm in its functional.py but doesn't declare scipy as a dependency, causing import errors throughout the xturing codebase.

Solution

Added explicit scipy dependency to both pyproject.toml and requirements-dev.txt to ensure scipy is available when bitsandbytes needs it.

Test plan

• Verified bitsandbytes import works with scipy installed
• Confirmed xturing modules can be imported without errors
• Test suite now runs successfully (tests/xturing/datasets/test_text_dataset.py passes)
• Pre-commit hooks passed