DOC/BLD: doc building with python 3
Makes our doc building machinery (specific accessor stuff) python 3 compat + switches travis doc building to python 3.5 + preventing some other warnings (see individual commits)
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15012?src=pr) is 84.78% (diff: 100%)
> Merging [#15012](https://codecov.io/gh/pandas-dev/pandas/pull/15012?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15012   diff @@
==========================================
  Files           145        145          
  Lines         51090      51090          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43315      43316     +1   
+ Misses         7775       7774     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d097b0f...a932919](https://codecov.io/gh/pandas-dev/pandas/compare/d097b0f0ed79a2305bc5970f45f01e768ac0c00a...a9329199477cc8ae8bf425f238e72d7f3b3822c3?src=pr) I only get errors installing the packages on travis:

```
UnsatisfiableError: The following specifications were found to be in conflict:
  - ipython
  - python 3.5*
```

anybody knows what the reason of this could be? It's beatiful-soup -> beautifulsoup4 apparently