Support Python 3.14: classifiers, docs, and CI matrix fix

This PR addresses issue #298 by completing Python 3.14 support: - Add Programming Language Python 3.14 classifier in setup.py. - Update docs docs/index.rst to say “supports CPython 3.9 up to 3.14”. - Fix a CI workflow typo so job python-version uses matrix.config[0], ensuring 3.14 runs as intended. - Note these changes under “8.1 (unreleased)” in CHANGES.rst.