BUG: Ensure .astype doesn't use PandasArray
On 0.24.0rc1, it's possible to end up with a PandasArray in internals.

```python
In [8]: ser = pd.Series([1, 2])

In [9]: ser.astype(ser.array.dtype)._data.blocks[0]
Out[9]: ExtensionBlock: 2 dtype: int64
```
