Deprecate is_extension_type
In favor of is_extension_array_type. This is blocked till sparse, period, and datetimetz are all EAs.

---


This should probably be True

```python
In [3]: pd.api.types.is_extension_type(pd.interval_range(0, 10))
Out[3]: False
```

Likewise for Period?

```python
In [3]: pd.api.types.is_extension_type(pd.period_range(2017, periods=2))
Out[3]: False
```

Currently we hardcode checks for categorical, sparse, and datetime w/ tz. A general solution would be to have all our extension types inherit from some base class or satisfy the interface required for an extension type.

xref https://github.com/pandas-dev/pandas/issues/18767