util: Make cpt_upgraders python3 compatible

util: Make cpt_upgraders python3 compatible

It won't be possible to build gem5 in a python3 only environment
otherwise since the cpt_upgrader.py script is automatically run at the
end of compilation

Change-Id: Iea4217e5cd64ca44b99aa0db5dabfdced7aeb3ea
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28587
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>