scons: Remove Python 2.7 compatibility code

scons: Remove Python 2.7 compatibility code

Remove the dependency on six and most 'import x from __future__'. A
few instances of imports from the future have been left in place to
ensure that Python 2.7 users still get an error message when invoking
scons.

Change-Id: I366275a6040f0084e91198b5b5c2a648bffbf2d2
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39585
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>