Migrate packaging from setup.py to pyproject.toml

[Copilot]

Replaces the monolithic setup.py with a pyproject.toml (PEP 517/518/621), moving all package metadata and configuration to the standard modern format.

Changes

• pyproject.toml (new): Full package metadata — name, version, description, authors, license, classifiers, dependencies, optional extras (testing, doc), src-layout package discovery, package data, and pytest config ([tool.pytest.ini_options])
• setup.py (stripped): Reduced to only the C extension definition (spectrum.mydpss), which setuptools still requires in setup.py
• .github/workflows/pypi.yml: Replaced deprecated python setup.py sdist with python -m build

Cleanup

• Removed EOL Python version classifiers (2.7, 3.5, 3.7) that conflicted with requires-python = ">=3.8"
• Removed redundant plot optional-dependency group (matplotlib is already a core dependency)

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[coveralls]

coverage: 97.016%. remained the same
when pulling d8cbc2f on copilot/update-to-pyproject-format
into c910058 on main.