Migrate packaging to pyproject.toml with hatchling

[endolith]

Closes #32.

Summary

• Add pyproject.toml with PEP 621 project metadata and [build-system] using hatchling as the build backend (per current PyPA guidance on moving away from setup.py).
• Declare runtime dependencies (numpy, scipy) and a dev extra (pytest, pytest-cov, flake8) so pip install -e ".[dev]" matches prior CI behavior without a separate requirements.txt.
• Remove setup.py and requirements.txt to avoid duplicated dependency sources.
• Update the GitHub Actions workflow to install the project and dev tools in one step.
• Refresh the README installation section for the new install paths.

Verification

• python3 -m build (wheel and sdist) succeeds locally.
• python3 -m pytest passes (104 passed, 9 skipped) after pip install -e ".[dev]".

  

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