CLN/DEPR: remove deprecated pandas.rpy module (GH9602)
Closes #9602
just a doc comment, otherwise lgtm. good to go? it's nice to update ecosystem doc to describe rpy2 and PypeR. @sinhrks do you know how maintained PypeR is, and whether pandas is supported well or not?
 @jorisvandenbossche their website is below, but can't find actual repo (looks to be not maintainanced 2014)? 

http://www.webarray.org/softwares/PypeR/

The package has ``use_pandas`` to handle ``DataFrame`` and ``Series``. From their source impl, ``rpy2`` is expected to work better.

```
        use_pandas: Used as a boolean value. A False value will disable pandas
            even if it has been imported. 
```

Pls drop ``PypeR`` description if it is not appropriate.

 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15142?src=pr) is 85.97% (diff: 100%)
> Merging [#15142](https://codecov.io/gh/pandas-dev/pandas/pull/15142?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.28%**

```diff
@@             master     #15142   diff @@
==========================================
  Files           145        140     -5   
  Lines         51430      51244   -186   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          44070      44057    -13   
+ Misses         7360       7187   -173   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [2fa33fb...e9d2d10](https://codecov.io/gh/pandas-dev/pandas/compare/2fa33fbddf4ae40b2dd47e146444a41bd6bccd6e...e9d2d109161ae79ed518a78c19890bcdeaa4238a?src=pr) http://pandas-docs.github.io/pandas-docs-travis/r_interface.html#rpy

something not defined here? Hmm, sigh. This worked locally, but I now see this only worked because I didn't activate pandas2ri. But if I don't activate pandas2ri, the second example does not work. I can't really get my head around rpy2's conversion/activate/dispatch rules and object model .. 
(will do a PR to fix this)