DEPR: deprecate pandas.rpy (GH9602)
closes #9602 

@jorisvandenbossche pls have a look (failures are the Mysql issue #9608)
 For me it is OK to already deprecate it, but I think we still should make sure that all existing functionality in pandas is ported to rpy2 (if any, but is should be checked)
 And, also provide in the docs some explanation about how to move from the pandas api to rpy2
 @jorisvandenbossche agreed. Nothing has really changed. But I think for example having `rpy2` add Categorical is a good idea.
 ok that we keep the #9602 issue open for this
