Fix concurrency bug in `FutureDependencyExecutor`

[gpauloski]

Description

The Python 3.12 tests were deadlocking in CI. This turned out to be the proble,

See the commit history for full details. I also made tests in CI run in verbose mode, and found a test dir with a missing __init__.py. Both of these were attempts to find the true source of the test deadlocks---the FutureDependencyError.

Fixes N/A

Type of Change

• [x] Bug (non-breaking change which fixes an issue)
• [ ] Enhancement (non-breaking change which adds or improves functionality)
• [ ] Internal (refactoring, performance, and testing)
• [ ] Breaking (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (no changes to the code)
• [ ] Development (CI workflows, packages, templates, etc.)
• [ ] Package (package dependencies and versions)

Testing

N/A

Pull Request Checklist

Please confirm the PR meets the following requirements.

• [x] Relevant tags are added (breaking, bug, documentation, enhancement, package, etc.).
• [x] Code changes pass pre-commit (e.g., ruff, mypy, etc.).
• [x] Tests have been added to show the fix is effective or that the new feature works.
• [x] New and existing unit tests pass locally with the changes.
• [x] Docs have been updated and reviewed if relevant.