feat: Drop support for python 3.9

[keelerm84]

Python 3.9 was EOL as of October 31st, 2025.

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Note

Raises the minimum supported Python version to 3.10 and updates CI, release workflows, and docs accordingly.

• CI matrices in ci.yml (Linux/Windows) now test Python 3.10–3.13 (removed 3.9)
• Publishing/release workflows (manual-publish.yml, release-please.yml) pin setup to Python 3.10 (was 3.9)
• pyproject.toml: set python = ">=3.10", drop 3.9 classifier, and set [tool.mypy] python_version = "3.10"
• README.md: Supported versions updated to Python 3.10+

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