DOC: avoid warnings ipython 'Code input with no code'
Need to check in the doc build log if this actually solves some of the warnings like

```
/home/travis/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/IPython/sphinxext/ipython_directive.py:1020: UserWarning: Code input with no code at /home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.24.0.rst, line 304
```

@datapythonista one annoying thing with the included `{{ header }}` is that the reported line numbers no longer are correct ... 