build: Remove legacy setup.py and modernize build tooling

[kunaaaalcodes]

PR Description

Summary

Fixes #470. Completes the build system migration started in #418 (scikit-build-core + CMake) and #458 (version.py as single source of truth) by removing all remaining legacy tooling and updating the documentation to match the modern workflow.

Problem

PR #418 migrated the build system to scikit-build-core + CMake, but the legacy 505-line setup.py remained in the repository alongside several files that still referenced outdated tooling:

• setup.py — dead code; scikit-build-core handles the build entirely
• scripts/ccache_g++ / scripts/ccache_clang++ — obsolete wrapper scripts; CMakeLists.txt already detects and wires up ccache natively via find_program(CCACHE_PROGRAM ccache)
• tox.ini — referenced black and flake8, which have been replaced by ruff in CI
• CONTRIBUTING.md — documented the old setup.py workflow, ccache wrapper scripts, and a --no-setup-py flag that no longer exists in pybind11-stubgen

Solution

1. Deleted setup.py (505 lines) — confirmed no remaining code references it.
2. Deleted scripts/ccache_g++ and scripts/ccache_clang++ — CMake handles ccache natively; CI configures ccache via CC/CXX env vars.
3. Made pedalboard/version.py the single source of truth for versioning — removed the hardcoded version = "0.9.24" from pyproject.toml, added dynamic = ["version"], and wired up a [[tool.dynamic-metadata]] block using scikit-build-core's built-in regex provider pointed at version.py. No new dependencies required.
4. Modernized tox.ini — replaced the dead black/flake8/format envs with ruff envs that mirror the CI checks in .github/workflows/all.yml (ruff check pedalboard, ruff format --check pedalboard --diff), and removed the obsolete [flake8] config section.
5. Updated CONTRIBUTING.md — replaced python3 setup.py build develop with pip install -e . --config-settings=cmake.build-type=Debug, simplified the ccache docs to reflect automatic detection, fixed the setup.py reference to point at CMakeLists.txt, and dropped the removed --no-setup-py flag.

Result

• Version bumps are now a one-line change in pedalboard/version.py
• Local and CI builds use the same scikit-build-core + CMake path
• Documentation and tox config no longer reference removed tooling
• 564 lines of dead/legacy code removed (505 in setup.py alone)

Verification

• pyproject.toml parses as valid TOML
• The [[tool.dynamic-metadata]] entry resolves to the correct version (0.9.24) from version.py using scikit-build-core's metadata resolution
• Ruff commands match CI's lint job byte-for-byte

References

• Issue: build: Complete removal of legacy setup.py and modernize build instructions #470
• Switch to CMake. #418 — Switch to CMake (scikit-build-core)
• Make version.py the single source of truth for version. #458 — Make version.py the single source of truth