Use the latest supported version of Python

[jsf9k]

🗣 Description

This pull request modifies Dockerfile to use a base Docker image based on the latest supported version of Python. We are currently limited to Python <= 3.7 because of cisagov/pshtt.

This pull request also bumps the base Docker image to be one based on Bullseye.

💭 Motivation and context

The latest version of Python is generally more performant than previous versions; plus, it's always good to stay current.

🧪 Testing

All automated tests pass.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.

✅ Post-merge checklist

• [x] Create a release.