chore: add Python version classifiers to fix PyPI version badge

[Copilot]

The [![Python](https://img.shields.io/pypi/pyversions/flameconnect)](https://pypi.org/project/flameconnect/) badge shows "missing" because shields.io reads Programming Language :: Python :: X.Y trove classifiers from PyPI metadata — not requires-python.

Changes

• pyproject.toml: Add classifiers with Programming Language :: Python :: 3 and Programming Language :: Python :: 3.13 to match the existing requires-python = ">=3.13" constraint

classifiers = [
    "Programming Language :: Python :: 3",
    "Programming Language :: Python :: 3.13",
]

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