Remove uses of six and imports from __future__ and use native Python 3 functionality instead. Change-Id: Ifeb925c0b802f8186dd148e382aefe1c32fc8176 Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com> Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39580 Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com> Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com> Tested-by: kokoro <noreply+kokoro@google.com>