Solve python2 vs. python3 problem on jenkins slaves

Usage of python2 as a default python version on our jenkins slaves causes lot of troubles. It would be good to use python3 everywhere.
e.g. lot of problem2 with openstack client on python2:
/usr/lib/python2.7/site-packages/pkg_resources/py2_warn.py:21: UserWarning: Setuptools will stop working on Python 2
************************************************************
You are running Setuptools on Python 2, which is no longer
supported and
>>> SETUPTOOLS WILL STOP WORKING <<<
in a subsequent release (no sooner than 2020-04-20).
Please ensure you are installing
Setuptools using pip 9.x or later or pin to `setuptools<45`
in your environment.
If you have done those things and are still encountering
this message, please follow up at
https://bit.ly/setuptools-py2-warning.
************************************************************
sys.version_info < (3,) and warnings.warn(pre + "" * 60 + msg + "" * 60)
Traceback (most recent call last):
File "/usr/bin/openstack", line 5, in <module>
from openstackclient.shell import main
File "/usr/lib/python2.7/site-packages/openstackclient/shell.py", line 24, in <module>
from osc_lib import shell
File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 33, in <module>
from osc_lib.cli import client_config as cloud_config
File "/usr/lib/python2.7/site-packages/osc_lib/cli/client_config.py", line 18, in <module>
from openstack.config import exceptions as sdk_exceptions
File "/usr/lib/python2.7/site-packages/openstack/_init_.py", line 16, in <module>
import openstack.config
File "/usr/lib/python2.7/site-packages/openstack/config/_init_.py", line 17, in <module>
from openstack.config.loader import OpenStackConfig # noqa
File "/usr/lib/python2.7/site-packages/openstack/config/loader.py", line 33, in <module>
from openstack.config import cloud_region
File "/usr/lib/python2.7/site-packages/openstack/config/cloud_region.py", line 44, in <module>
from openstack import proxy
File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 24, in <module>
from openstack import resource
File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 49, in <module>
from openstack import utils
File "/usr/lib/python2.7/site-packages/openstack/utils.py", line 13, in <module>
import queue
ImportError: No module named queue