Find a way to detect whether builtin/extension classes have `__dict__`

Some builtin/extension classes have a `__dict__`...

```python
>>> RuntimeError().__dict__
{}
```

...while others do not

```python
>>> list().__dict__  # error
>>> Decimal().__dict__  # error
```

I haven't found a way to be able to distinguish these, without instantiating an instance.