configs,python: Fixes an issue with python3 and the config scripts when restoring...

configs,python: Fixes an issue with python3 and the config scripts when restoring a checkpoint

Fixes a compatibility issue with the configuration scripts when trying to restore a checkpoint.  Since python2.4 list.sort has an updated interface.  The older one has been dropped in python3.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-754

Change-Id: I09f819057d510e477d6ceae0356fafad40f4280d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34295
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>