0.24.0 DateTimeIndex: union() got an unexpected keyword argument 'sort'
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
index1 = pd.DataFrame(index=pd.to_datetime(['19900101'])).index
index2 = index1.copy()
index1.union(index2, sort=False)
TypeError: union() got an unexpected keyword argument 'sort'
```
#### Problem description
The 'sort' argument works with 'difference' and 'intersection' but fails on 'union' for DateTimeIndex.
