DOC: Recommend sphinx 1.5 for now
xref https://github.com/pandas-dev/pandas/issues/16705

I've started to debug the slowdown (it's on the writing side), but haven't diagnosed the cause. For the SciPy sprints tomorrow we should recommend 1.5
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=h1) Report
> Merging [#16929](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6000c5b9624fdd8925099f215eba282bfbef87ce?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/16929/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16929      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49303    49303              
==========================================
- Hits        44863    44854       -9     
- Misses       4440     4449       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16929?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/16929?src=pr&el=footer). Last update [6000c5b...b471e54](https://codecov.io/gh/pandas-dev/pandas/pull/16929?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
