Revert "DOC: Remove vendored IPython.sphinext (#18193)"
This reverts commit ca737aca8b73b10b617f7fcc7da35545f9ec0a89.

This reverts https://github.com/pandas-dev/pandas/pull/18193. The problem is there is something wrong with the upstream version such that errors in the code don't get reported. 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=h1) Report
> Merging [#18320](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/54f2a5e91e90e35f7cbd15214297169831d6a6a6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18320/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18320      +/-   ##
==========================================
- Coverage   91.39%   91.37%   -0.02%     
==========================================
  Files         164      164              
  Lines       49808    49808              
==========================================
- Hits        45520    45511       -9     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <ø> (ø)` | :arrow_up: |
| #single | `39.47% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18320?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/18320?src=pr&el=footer). Last update [54f2a5e...7f1cc25](https://codecov.io/gh/pandas-dev/pandas/pull/18320?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger I identified the source of the problem: https://github.com/ipython/ipython/pull/9823 So another option is to pin IPython to an older version (although I would maybe rather vendor the latest upstream version + bug fix, to mimic that we are using the latest IPython version) I opened an issue at IPython: https://github.com/ipython/ipython/issues/10904 (with potential solution) Thanks for tracking it down @jorisvandenbossche. I think reverting https://github.com/pandas-dev/pandas/commit/ca737aca8b73b10b617f7fcc7da35545f9ec0a89 for now is fine. And then once the next IPython is released with your fix, we'd require it for building the docs.