Add Python 3.14 support

[MaxGhenis]

Summary

This PR adds support for Python 3.14 to policyengine-us.

Changes

• Updated requires-python to >=3.10,<3.15 to include Python 3.14
• Added Python 3.14 to classifiers in pyproject.toml
• Updated all CI workflows (pr.yaml and push.yaml) to use Python 3.14

Dependencies

• Depends on policyengine-core supporting Python 3.14 (PolicyEngine/policyengine-core#401)

Test Plan

• CI will run tests on Python 3.14
• All existing tests should pass

Related Issue

Fixes #6626

🤖 Generated with Claude Code

[MaxGhenis]

Status Update

Python 3.14 was released on October 7, 2025. CI tests are currently failing because h5py doesn't have Python 3.14 wheels yet.

Current Situation

• h5py has Python 3.14 wheels in nightly builds
• Stable release (h5py 3.15) is being planned (issue #2651)
• Building h5py from source is failing on Python 3.14

Next Steps

This PR will remain in draft status until h5py 3.15 is released on PyPI. Once that happens, CI should pass and the PR can be marked ready for review.

Tracking: h5py/h5py#2713