tests: enforce the use of `from __future__ import annotations` A recent MR and a separate recently landed MR that extracted code to a new file overlooked this, so I think it's worth flagging to ensure consistency. We don't enforce the import for empty files (like `__init__.py`). I'd rather this go into `import-checker.py`, but the import of interest only happens at the top of the file, and its `verify_modern_convention()` calls itself recursively as it transits the AST where the annotations might be. After a few hours of hacking on trying to get it to enforce the import, but only if annotations are used in the module (we generally don't have or check annotations in test files, so don't need this import), I gave up and resorted to this. It won't handle multi-line imports, but this isn't something I'd expect to change often, so this is good enough for now.