Officially support Python 3.10

[mcdonnnj]

🗣 Description

This pull request marks the Python package as officially supporting Python 3.10.

💭 Motivation and context

In #87 we added testing and building for Python 3.10 but did not mark the Python package as supporting Python 3.10 (oops!).

⚠ Note

This work is currently included in #90, but after thinking about it more I would like to enable Python 3.10 support and terminate 3.6 support as separate tasks (with different versioning).

🧪 Testing

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.

✅ Pre-merge checklist

• Finalize version.

✅ Post-merge checklist

• Add a tag or create a release.

[dav3r]

👍 🐍