Add OS constraint in PyPI

Mention MLServer's OS constraints as metadata in `setup.py`, so that it becomes visible in pypi.org. 

```
setup(...,
      classifiers=[
          'Operating System :: POSIX',
          ],
      )
```

_Originally posted by @HugoMVale in https://github.com/SeldonIO/MLServer/issues/1022#issuecomment-1456788132_
            