CI: add Python 3.13 and 3.14 to the test matrix (#360).
Validates the Python 3.14 argparse fix (nested argument groups are now a hard error) end-to-end on real 3.13/3.14 interpreters — see #318 / #359. No library code changes; the full test suite and pyensembl install pass on 3.9 through 3.14.
PyPI: https://pypi.org/project/pyensembl/2.10.2/
