modernise build system: Add pyproject.toml and migrate to PEP 517

[DininduSenanayake]

Summary

Modernizes the package build system by adding pyproject.toml and adopting PEP 517 standards to ensure compatibility with pip 25.3+ and eliminate deprecation warnings.

Changes

• Added pyproject.toml with declarative package metadata and dependencies
• Simplified setup.py to delegate configuration to pyproject.toml
• Updated installation method from python setup.py develop to pip install -e .

Why This Change Is Needed

1. Addresses deprecation warnings: pip is removing support for the legacy setup.py bdist_wheel mechanism in version 25.3
2. Future-proofs the package: Adopts the standardized build interface (PEP 517/518) that is now the Python packaging standard
3. Improves maintainability: Declarative configuration in pyproject.toml is easier to read and maintain than imperative setup.py
4. Better dependency management: Explicitly declares build system requirements

Testing

• Package installs successfully with pip install -e .
• All dependencies resolve correctly
• Entry point (cellhub command) works as expected
• No deprecation warnings during installation

Backward Compatibility

• Maintains setup.py for backward compatibility with older tools
• Does not change package functionality or API
• Only affects the build/installation process

Migration Notes for Users

Users should update their installation commands:

• Old: python setup.py develop
• New: pip install -e .

Both methods currently work, but the new method is recommended going forward.