Remove the dependency on six in testlib. Change-Id: I247088d119cf8f9d815632eae16a1cbf87930516 Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com> Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39759 Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu> Tested-by: kokoro <noreply+kokoro@google.com>