Don't overwrite identical wrappers

[kwabenantim]

Supports #74

[Copilot]

Pull request overview

Implements incremental wrapper generation by avoiding rewrites when regenerated wrapper content is identical, reducing downstream rebuilds, and adds a CLI escape hatch to force rewrites when needed (supports #74). The PR also modernizes packaging/CI by moving project metadata into pyproject.toml, adding unit-test CI, and adjusting declared Python support.

Changes:

• Add write_file_if_changed() and thread an overwrite flag through generator/writers; expose it via --overwrite.
• Update wrapper writers to skip writing unchanged wrapper files by default (preserving mtimes).
• Migrate packaging metadata from setup.cfg/setup.py to pyproject.toml, add unit tests + unit-test workflow, and update docs/CI configuration accordingly.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_utils.py	Adds unit tests for “write only if changed” behavior and overwrite forcing.
setup.py	Removes legacy setuptools entrypoint (packaging migrated to pyproject.toml).
setup.cfg	Removes legacy package metadata/config (packaging migrated to pyproject.toml).
README.md	Documents the new --overwrite behavior and incremental rewrite default.
pyproject.toml	Adds PEP 621 project metadata / extras, updates build backend, and changes declared Python requirement.
examples/cells/pyproject.toml	Updates example package’s declared Python requirement.
examples/cells/conda/variants/python3.9.yaml	Removes Python 3.9 conda variant.
examples/cells/conda/variants/python3.8.yaml	Removes Python 3.8 conda variant.
cppwg/writers/package_writer.py	Threads overwrite setting from package writer into module writers.
cppwg/writers/module_writer.py	Uses write_file_if_changed() for module wrapper output and propagates overwrite into class writer.
cppwg/writers/header_collection_writer.py	Uses write_file_if_changed() for header collection output.
cppwg/writers/class_writer.py	Uses write_file_if_changed() for per-class .hpp/.cpp outputs.
cppwg/utils/utils.py	Introduces write_file_if_changed() helper used by writers.
cppwg/generators.py	Adds overwrite option to generator and passes it through to writers.
cppwg/__main__.py	Adds --overwrite CLI flag and forwards it to the generator.
.github/workflows/test-unit.yml	Adds a unit-test workflow running pytest.
.github/workflows/test-style.yml	Expands workflow triggers to run on develop pushes.
.github/workflows/test-shapes-pip.yml	Expands workflow triggers to run on develop pushes.
.github/workflows/test-cells-ubuntu.yml	Expands workflow triggers to run on develop pushes.
.github/workflows/test-cells-conda.yml	Expands workflow triggers to run on develop pushes.

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