Cannot use .ix in IntervaIndex('pandas._libs.interval.IntervalTree' object has no attribute 'get_value')
#### Code Sample, a copy-pastable example if possible


```python
x = pd.Series([-2.801298, -2.882724, -3.007899, -2.704554, -3.398761, -2.805034, -2.87554, -2.805034, -2.886459, -2.471618])
y= pd.Series([0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
init_cut = pd.qcut(x, 5, duplicates='drop')
retbin = pd.Series(init_cut.values.categories).sort_values()
retbin.iloc[0] = pd.Interval(-np.inf, retbin.iloc[0].right)
retbin.iloc[-1] = pd.Interval(retbin.iloc[-1].left, np.inf)
init_cut = pd.cut(x, pd.IntervalIndex(retbin))
init_cut = init_cut.astype(object)
bin_df = pd.crosstab(index=init_cut, columns=y)
bin_df = bin_df.reindex(retbin)
bin_df = bin_df.sort_index()
bin_df = bin_df.fillna(0.0)
bin_df['nbin'] = np.nan
```

#### Problem description
bin_df = 
col_0             0  nbin
(-inf, -2.911]    2   NaN
(-2.911, -2.878]  2   NaN
(-2.878, -2.805]  3   NaN
(-2.805, -2.782]  1   NaN
(-2.782, inf]     2   NaN


if I use bin_df.ix[0:2,0], I got an error like:

```pytb
Traceback (most recent call last):
  File "D:\anaconda\lib\site-packages\IPython\core\interactiveshell.py", line 2961, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-12-1ae8ba69565c>", line 1, in <module>
    bin_df.ix[0:1,'nbin']
  File "D:\PyTest\venv\lib\site-packages\pandas\core\indexing.py", line 125, in __getitem__
    values = self.obj._get_value(*key)
  File "D:\PyTest\venv\lib\site-packages\pandas\core\frame.py", line 2827, in _get_value
    return engine.get_value(series._values, index)
AttributeError: 'pandas._libs.interval.IntervalTree' object has no attribute 'get_value'
```


the version is 0.25.0
but it works well in 0.24.x

#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

</details>
