Pin MongoEngine

[king-alexander]

🗣 Description

I pinned MongoEngine to a version between 0.16.3 and 0.27.0.

💭 Motivation and context

MongoEngine 0.27.0 introduces a breaking change to the Mongomock connection method. This breaks our tests that connect to a mock database.

🧪 Testing

I rebuilt the admiral image with the new MongoEngine requirements. All tests passed locally.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• 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

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[dav3r]

🚀