Refactor Packaging: Split setup.py into setup.py and pyproject.toml for PEP 517 Compliance

[michael-mue]

Description

This PR splits the setup.py file into setup.py and pyproject.toml to adhere to PEP 517 and modern packaging standards. This change facilitates better build isolation, dependency specification, and aligns with the latest Python packaging practices.

Changes made

• Created a pyproject.toml file containing build system requirements and project metadata.
• Simplified setup.py to focus on building extensions and registering custom commands.
• Ensured that all existing setup configurations are appropriately transferred to pyproject.toml.
• Updated README with new install from source instructions

Motivation

• Aligns cornac with modern packaging standards and simplifies dependency installation.
• Enhances compatibility with tools like uv and pixi that resolve dependencies simultaneously.
• Improves compatibility with tools like pip and build that rely on pyproject.toml.
• Simplifies the source installation process by eliminating the need to pre-install build dependencies (pip3 install git+https://github.com/PreferredAI/cornac.git works without requiring pip3 install Cython numpy scipy).
• Improves build reproducibility and isolation.

Testing

• All tests pass locally.
• Built the package locally using the new configuration without issues.

Notes

• No changes to the core functionality of the package.
• All existing tests and CI configurations should remain valid.

Please let me know if there are any questions or if further changes are needed.