v1.0.16

Add Python 3.15 support, set version to v1.0.16

- Raise python_requires cap to <3.16 and drop the unsupported <3.9 floor
- Build with eMudhra-hardened gha-cibuildwheel (cibuildwheel v4.3.3)
- Remove the cpython-freethreading enable option, rejected by cibuildwheel
  v4; 3.14+ free-threaded builds are on by default. cp313t wheels are no
  longer built.
- Remove the publish_pypi workflow; PyPI uploads go through wheelmonger
- Fix --iterations typo in the free-threading parallel test run