Consider adding ExtensionDtype.is_dtype_equal
We have `is_dtype` for checking whether the dtype / array's dtype is of the same kind as `self`. It may sometimes be useful to have an *exact* comparison for parametrized dtypes like `Categorical` and `Period`.

So

```python
>>> PeriodDtype('4D').is_dtype(PeriodDtype('5D'))
True
>>> PeriodDtype('4D').is_dtype_equal(PeriodDtype('5D'))
```