Usability: TAB completion no longer works with accessors
When doing the following in the IPython console:

```
In [1]: pd.Series.str.<TAB>
```

or 

```
In [2]: s = pd.Series(['a', 'b']) 

In [3]: s.str.<TAB>
```

this used to give the list of available methods on the `str` accessor (the same for eg `dt`). But with latest master / latest IPython this no longer works. This is quite a usability regression, I typically used it when giving workshows to show a way to interactively check what methods are available.

The non working case is with pandas 0.24.2 + IPython 7.5.0. I checked it with another environment with pandas 0.22.0 + IPython 5.8.0 where it is working. 
So it could be either due to a change in pandas or a change in IPython (or one of its dependencies for this).