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 ... 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=h1) Report
> Merging [#24238](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6c7f6be80c83ad9f4680fc7bcd5bf9e5b426d92?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24238/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24238   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51763    51763           
=======================================
  Hits        47733    47733           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=footer). Last update [f6c7f6b...644842a](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=h1) Report
> Merging [#24238](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6c7f6be80c83ad9f4680fc7bcd5bf9e5b426d92?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24238/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24238   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51763    51763           
=======================================
  Hits        47733    47733           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=footer). Last update [f6c7f6b...644842a](https://codecov.io/gh/pandas-dev/pandas/pull/24238?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > Seems like the {header} is creating several problems, but I think the only alternative is having the same statements at the beginning of each file, and in my opinion that is worse than the problems so far. Any other idea?

No, not really another idea. It's indeed duplicated content, but on the other hand, also some lines that are hardly ever touched I think. So personally I don't think that this duplication is that of a problem. 
I can just say that fixing warnings of sphinx is annoying in general, having the line numbers not match up is not making it better :-)

 Absolutely agree. Will see if I can monkeypatch sphinx. I'm finishing a PR to build a single documentation page faster, and fail if it has warnings, so we can start fixing them in an efficient way. So would be great to have the line numbers matching, as we'll have to fix plenty of them.