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.