support/docker: add python3

support/docker: add python3

The test infra will soon be converted to Python 3 only.
So add the interpreter and also the Python 3 variant of modules nose2
and pexpect to the docker image used to run runtime tests.

Keep the Python 2 variant of those modules to allow a gradual
transition.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Nicolas Carrier <nicolas.carrier@orolia.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>