cran: Use pyreadr instead of rpy2 to read a RDS file from Python
The CRAN lister improvements introduced in 91e4e33d originally used pyreadr to read a RDS file from Python instead of rpy2.
As swh-lister was still packaged for debian at the time, the choice of using rpy2 instead was made as a debian package is available for it while it is not for pyreadr.
Now debian packaging was dropped for swh-lister we can reinstate the pyreadr based implementation which has the advantages of being faster and not depending on the R language runtime.
Related to swh/meta#1709 (closed).