DataFrame.sort_index() with ascending passed as a list on a single level index gives wrong result
I found some problems while using data_frame sort_index,  first, create dataframe

```
data = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]), columns=['a', 'b', 'c'])
data = data.set_index('a')
data.sort_index(level=['a'], ascending=[False])
```
Then I want to sort index "a" in descending order,
```
data.sort_index(level=['a'], ascending=[False])
```
The above code does not sort index a in descending order,   I found the reason is that this dataframe is a single index structure, so when ascending = list is passed, it does not take effect

https://github.com/pandas-dev/pandas/blob/bf613c14f41b624b432d0d6b9a29007e7990d460/pandas/core/frame.py#L4777-L4783

Because when it is used, the index may be single index or multi-index. In order to use the effect uniformly,I think there should be a judgment here. If index is a single index and ascending type == list, ascending should be equal to ascending [0]，This may be more user friendly

If yes, can I mention pr