QuadPrecision Scalar is not exposing dtype correctly

```python
In [2]: QuadPrecision("1e100", backend="longdouble").dtype
Out[2]: QuadPrecDType(backend='sleef')

In [3]: np.dtype(QuadPrecision("1e100", backend="longdouble"))
Out[3]: QuadPrecDType(backend='sleef')
```