feat: Add Python 3.14 support

[bhimrazy]

What does this PR do?

Adds support for Python 3.14 across the Lightning Utilities repository.

Changes Made

• setup.py: Added Python 3.14 classifier
• CI workflows: Updated testing matrices to include Python 3.10-3.14
• ci-testing.yml: Refactored to use include for oldest requirements testing on 3.10-3.12

Testing Strategy

• All Python versions (3.10-3.14) tested with latest dependencies
• Oldest dependencies tested on Python 3.10-3.12 only for CI efficiency

PR review

Anyone in the community is free to review the PR once the tests have passed.

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📚 Documentation preview 📚: https://lit-utilities--464.org.readthedocs.build/en/464/