Upgrading an instance containing plpython from <=8.4 to >=9.0 is broken
because the module plpython.so was renamed to plpython2.so, and so the
pg_upgrade check for loadable libraries fails thus:
Your installation references loadable libraries that are missing from the new installation. etc.
Installing a symlink fixes the issue. Should we teach pg_upgrade about
this renaming, or should we install the symlink as part of the standard
installation?