Force python3 for zuul-tarball job
Since both zuul and nodepool only support python3, use python3 to build
sdist and bdist_wheel.
tasks:
- name: Build tarball and wheel
command:
- cmd: python setup.py sdist bdist_wheel
+ cmd: python3 setup.py sdist bdist_wheel
chdir: "{{ zuul.project.src_dir }}"
