Replace deprecated license classifier with SPDX package metadata #83

The 2.9.0 release succeeded, but ./deploy.sh emitted repeated setuptools warnings during the sdist/wheel build:

WARNING License classifiers are deprecated.
Please consider removing the following classifiers in favor of a SPDX license expression:
License :: OSI Approved :: Apache Software License

pyproject.toml currently declares the Apache license only through that classifier. Migrate to the appropriate SPDX license metadata and ensure the build-system setuptools minimum supports it. Verify the generated wheel/sdist metadata and distribution checks without changing the project's license.

This is packaging maintenance; it did not block publication or the installed-package smoke test. Found while deploying #82. Related cleanup: #76.