DEPR: special-casing dt64/td64 in Series.unique
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

```
dti = pd.date_range('2016-01-01', periods=3).repeat(2)
ser = pd.Series(dti)

>>> ser.unique()
<stdin>:1: FutureWarning: Series.unique behavior with datetime64[ns] dtype is deprecated. In a future version this will return a <class 'pandas.core.arrays.datetimes.DatetimeArray'> instead of a np.ndarray
array(['2016-01-01T00:00:00.000000000', '2016-01-02T00:00:00.000000000',
       '2016-01-03T00:00:00.000000000'], dtype='datetime64[ns]')

```