Pin setuptools to avoid mlflow import failure (pkg_resources missing)

`mlflow==2.10.0` imports `pkg_resources`, but our environment currently resolves `setuptools` to latest, where `pkg_resources` may be unavailable. This causes:
`ModuleNotFoundError: No module named 'pkg_resources'`

Easy fix is to add `setuptools<81` to requirements.txt

This triggers a deprecation warning:

```
UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html.
The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
import pkg_resources # noqa: TID251
```

But works fine