arch, mem, cpu, systemc: Remove Python 2.7 glue code

arch, mem, cpu, systemc: Remove Python 2.7 glue code

Remove uses of six and from __future__ imports as they are no longer
needed.

Change-Id: Ib10d01d9398795f46eedeb91a02736f248917b6a
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39758
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>