gitlab/travis: Rework how and when we use virtualenv in order to use python3

As things stand today, we have tools that CI requires where "python"
must be "python2".  We need to use a virtualenv and pip in order to
ensure that our pytest tests can be run.  Rework things slightly so
that:
- On Travis-CI, we install python-pyelftools for the platforms that
  require pyelftools to be installed.
- On GitLab-CI, we move to a newer base image that includes python3-pip
  and continue to use a virtualenv per job that needs it, for the
  correct set of packages.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>