[codex] Update Python support to 3.14

[mtinning]

Summary

Updates PyQtToolkit to target Python 3.14, including a .python-version pin for Python 3.14.6, packaging metadata, tox, and CircleCI images.

Details

• Sets python_requires to >=3.14.
• Moves tox and CircleCI from Python 3.6 to Python 3.14.
• Replaces the unmaintained licensify test dependency with an in-repo license header checker because licensify no longer builds on Python 3.14.
• Patches bundled Versioneer config parsing for Python 3.14 compatibility.
• Fixes test/runtime issues exposed by Python 3.14 and current pytest-qt/PyQt behavior.

Validation

• QT_QPA_PLATFORM=offscreen .venv/bin/python -m pytest tests
• .venv/bin/tox -e py314

[mtinning]

lgtm