Remove Python 3.9 support

[jsf9k]

🗣 Description

This pull request drops support for Python 3.9.

💭 Motivation and context

Python 3.9 is EOL as of October 31, 2025.

🧪 Testing

All automated tests pass.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.
• Bump major, minor, patch, pre-release, and/or build versions as appropriate via the bump_version script if this repository is versioned and the changes in this PR warrant a version bump.
• Create a pre-release (necessary if and only if the pre-release version was bumped).

✅ Pre-merge checklist

• Finalize version.
• Mark Python 3.9 GHA checks as no longer required.

✅ Post-merge checklist

• Create a release (necessary if and only if the version was bumped).