Remove uses of six and imports from __future__ and use native Python 3 functionality instead. Change-Id: If37718ba99def2d6f176604e20d4ebeda75474ad Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com> Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39581 Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com> Reviewed-by: Richard Cooper <richard.cooper@arm.com> Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com> Tested-by: kokoro <noreply+kokoro@google.com>