This ticket adds support for rpy 3.x while keeping support for rpy 2.x.

This is done by defining rpy2py as cv.rpy2py for rpy 3.x and as cv.ri2py for rpy 2.x.

CC: @slel

Component: interfaces

Keywords: rpy

Author: Antonio Rojas

Branch/Commit: 6e67c65

Reviewer: Isuru Fernando

Issue created by migration from https://trac.sagemath.org/ticket/28988