CLN: removed pandas.sandbox
Deprecated back in `0.16.0` <a href="https://github.com/pydata/pandas/pull/9615">here</a>, and the module seems to be rarely, if at all, used according to @jorisvandenbossche .

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13670?src=pr) is **84.51%**

> Merging [#13670](https://codecov.io/gh/pydata/pandas/pull/13670?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.13%**

``` diff
@@             master     #13670   diff @@
==========================================
  Files           142        141     -1   
  Lines         51223      51142    -81   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43224      43224          
+ Misses         7999       7918    -81   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [ada6bf3...5a397a2](https://codecov.io/gh/pydata/pandas/compare/ada6bf350f7fd4daaf2b80188ca165fb9543a252...5a397a229bbb480fae0e25b46c04860ae363ab7c?src=pr)
 @sinhrks : made the requested doc comments (no need to run Travis tests again hence the `[ci skip]`), so ready to merge if there are no other concerns.
 thank you sir!
