DOC: increase recursion limit on sphinx builds
xref https://github.com/pandas-dev/pandas/pull/15637#issuecomment-285558452

hopefully should eliminate the non-deterministic pickle errors in doc-building :<
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15641?src=pr&el=h1) Report
> Merging [#15641](https://codecov.io/gh/pandas-dev/pandas/pull/15641?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/470c3276479925a198f38f9c0aacd745ef3a64bd?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15641      +/-   ##
==========================================
- Coverage   91.02%      91%   -0.03%     
==========================================
  Files         143      143              
  Lines       49324    49324              
==========================================
- Hits        44898    44887      -11     
- Misses       4426     4437      +11
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15641?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/470c3276479925a198f38f9c0aacd745ef3a64bd...3ac9d793845f34e74f87471b1b11fc1e1395af34?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/470c3276479925a198f38f9c0aacd745ef3a64bd...3ac9d793845f34e74f87471b1b11fc1e1395af34?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.76% <0%> (-0.35%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/470c3276479925a198f38f9c0aacd745ef3a64bd...3ac9d793845f34e74f87471b1b11fc1e1395af34?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15641?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/15641?src=pr&el=footer). Last update [470c327...3ac9d79](https://codecov.io/gh/pandas-dev/pandas/compare/470c3276479925a198f38f9c0aacd745ef3a64bd...3ac9d793845f34e74f87471b1b11fc1e1395af34?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). Great! That was indeed the error I saw locally (so not related to our pickle changes). I just didn't see it anymore afterwards, and on travis in the PR, but apparently it is not each time, or so.