update test workflow for modern python and github actions

[purificant]

Upgrade the CI test workflow (.github/workflows/tests.yml) by extending Python coverage and bumping pinned tooling.

• Expand test matrix to include Python 3.13 and 3.14
• Bump GitHub Actions: actions/checkout v4 to v6, actions/setup-python v4 to v6, actions/cache v4 to v5
• Bump Poetry from 1.8.3 to 2.3.4
• Bump the lint job's Python from 3.11 to 3.14 so linting runs on the newest interpreter

[gregsvo]

I'm so sorry for the delay- I didn't expect contributions as quickly, and I forgot to check! Thanks so much for this — keeping the CI matrix current with Python 3.13 and 3.14 is super great, and I appreciate you helping keep this project healthy. The Poetry 2.x bump and updated action versions are appreciated too. Merging this in!

[dandigangi]

Thanks for contributing @purificant! We appreciate it!