We noticed that `mlir/python/requirements.txt` lists `ml_dtypes` as a requirement but when looking at the code in `mlir/python`, the only `import` is guarded:
```python
try:
import ml_dtypes
except ModuleNotFoundError:
# The third-party ml_dtypes provides some optional low precision data-types for NumPy.
ml_dtypes = None
```
This makes `ml_dtypes` an optional dependency.
Some python tests however partially depend on `ml_dtypes` and should not run if that module is unavailable. That is what this change does.
This is a replacement for #123051 which was excluding tests too broadly.
34d50721db