Add Python 3.14 support

## Summary

Add official support for Python 3.14, which was released on October 7, 2025.

## Current Status

Currently, PyAthena supports Python 3.9 through 3.13.

## Required Changes

### 1. Update `pyproject.toml`
- Add `"Programming Language :: Python :: 3.14"` to classifiers
- Update tox configuration to include `py314` in envlist
- Add `3.14: py314` to gh-actions mapping

### 2. Update CI Workflow
- Add `'3.14'` to the test matrix in `.github/workflows/test.yaml`
- Check if Python 3.14 requires special setup like Python 3.13 does (setup-python workaround)

### 3. Testing
- Run full test suite with Python 3.14
- Verify all dependencies are compatible with Python 3.14
- Check for any deprecation warnings or compatibility issues

## Python 3.14 Major Features

Python 3.14 includes several significant improvements:
- Free-threaded Python official support (PEP 779)
- Deferred evaluation of annotations (PEP 649)
- Template string literals/t-strings (PEP 750)
- Multiple interpreters in the standard library (PEP 734)
- Experimental JIT compiler

## References

- [PEP 745 – Python 3.14 Release Schedule](https://peps.python.org/pep-0745/)
- [Python 3.14.0 Release](https://www.python.org/downloads/release/python-3140/)
- [What's New in Python 3.14](https://docs.python.org/3.14/whatsnew/3.14.html)

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