tests/docker: upgrade docker.py to python3

The recent podman changes (9459f754) imported enum which is part
of the python3 standard library but only available as an external
library for python2. This causes problems on the fairly restricted
environment such as shippable. Lets bite the bullet and make the
script a fully python3 one. To that end:

  - drop the from __future__ import (we are there now ;-)
  - avoid the StringIO import hack
  - be consistent with the mode we read/write dockerfiles
  - s/iteritems/items/
  - ensure check_output returns strings for processing

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>