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>