[Python APIView] Python public constants don't appear in APIView

Ran into a case when dealing with azure-core in the Python SDK:

In the `azure.core.serialization` namespace, we have a public constant that is commonly imported by other SDK packages:

https://github.com/Azure/azure-sdk-for-python/blob/b25a0309f5ffecb0fc4c16ba0e551bdaaa6560ff/sdk/core/azure-core/azure/core/serialization.py#L25

```python
NULL = _Null()
```

However, this constant does not appear in the APIView: https://apiview.dev/Assemblies/Review/998d0146014f4305bd4e6054b2eb56aa#azure.core.serialization

Would be great if constants in public namespaces that are not prefixed with an underscore show up in APIView. Or at least ones in the `__all__` list.