Fix Python 3.8/3.9 compatibility and action failures

[Copilot]

Summary of Changes

Fixed failing GitHub Actions caused by Python 3.10+ syntax incompatibility and missing test dependencies.

Key Changes:

• Replaced union syntax (np.ndarray | None) with Optional[np.ndarray] for Python 3.8+ compatibility
• Added missing hypothesis dependency to tox.ini
• Fixed import ordering violations (E402)
• Configured ruff to ignore assert statements and line length in test files
• Auto-formatted codebase with ruff

Files Modified:

• pymars/_forward.py, pymars/_pruning.py: Union syntax → Optional
• pymars/_basis.py, pymars/_sklearn_compat.py, tests/test_sklearn_compat.py: Import ordering
• tox.ini: Added hypothesis dependency
• pyproject.toml: Excluded S101, E501 from test files

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

• Ran existing tests to ensure no regressions
• Manual testing performed

Verified Python 3.12 can import fixed modules without errors. Full CI validation pending workflow execution.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Related Issues

Addresses action run failures on main branch.

Additional Context

Remaining Items:

• Mypy errors persist (missing type annotations, not runtime-critical)
• Codecov upload requires token configuration (admin action needed)

Both are non-blocking for core functionality.

Original prompt

There are failing actions. Can you review the action runs, and address the errors?

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