misc: Added python-six as a dependency in 18.04 docker

Previously the docker utilized Python-Pip. This is an intermediate
dependency we don't need. We can install the Python-six module directly
via the APT framework.

Change-Id: I30e3e1cdca802ca19422140f39af7dc9dc166ed7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28891
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed