IndexError: tuple index out of range after upgrade to 0.25
Root cause (in both cases using `df = pd.DataFrame({'a': [1, 2, 3]})`):

```
In [71]: pd.__version__  
Out[71]: '0.25.0'

In [73]: df.index[:, None]
Out[73]: Int64Index([0, 1, 2], dtype='int64')

In [74]: df.index[:, None].shape
Out[74]: (3,)
```

vs 

```
In [10]: pd.__version__  
Out[10]: '0.24.2'

In [13]: df.index[:, None] 
Out[13]: Int64Index([0, 1, 2], dtype='int64')

In [14]: df.index[:, None].shape
Out[14]: (3, 1)
```

So before, indexing with `[:, None]` (in numpy a way to add a dimension to get 2D array) actually resulting in Index with ndim of 2 (but which is of course inconsistent state of the Index object)

Matplotlib relied on this fact when an Index is passed to `plt.plot`, as reported in https://github.com/matplotlib/matplotlib/issues/14992

---

I have explained the issue [here ](https://stackoverflow.com/questions/57372557/matplotlib-drops-indexerror-tuple-index-out-of-range-after-upgrade)and [here ](https://github.com/matplotlib/matplotlib/issues/14992#issue-477267556)in details. Basically, after upgrading to the version `0.25` I got the error:

> IndexError: tuple index out of range

while attempting to plot a CSV file. 