overcloud-prep-containers : Update pip returns SyntaxError: invalid syntax

Hi,

Seen in that review https://review.opendev.org/c/openstack/tripleo-common/+/770767 for:

 - tripleo-ci-centos-7-scenario000-multinode-oooq-container-upgrades: https://zuul.opendev.org/t/openstack/build/8f69a60c44cc4821bb484ddb14ba48a1
 - tripleo-ci-centos-7-containers-multinode: https://zuul.opendev.org/t/openstack/build/312b125ed4034fc28200c596e3996380

    TASK [overcloud-prep-containers : easy-install pip] ****************************
    Monday 25 January 2021 18:28:42 +0000 (0:00:00.075) 0:28:17.379 ********
    changed: [undercloud]

    TASK [overcloud-prep-containers : Update pip] **********************************
    Monday 25 January 2021 18:28:51 +0000 (0:00:09.119) 0:28:26.498 ********
    fatal: [undercloud]: FAILED! => {
        "changed": false,
        "cmd": [
            "/bin/pip2",
            "install",
            "-U",
            "--upgrade",
            "pip"
        ]
    }

    MSG:

    :stderr: Traceback (most recent call last):
      File "/bin/pip2", line 9, in <module>
        load_entry_point('pip==21.0', 'console_scripts', 'pip2')()
      File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 542, in load_entry_point
        return get_distribution(dist).load_entry_point(group, name)
      File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2569, in load_entry_point
        return ep.load()
      File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2229, in load
        return self.resolve()
      File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2235, in resolve
        module = __import__(self.module_name, fromlist=['__name__'], level=0)
      File "/usr/lib/python2.7/site-packages/pip-21.0-py2.7.egg/pip/_internal/cli/main.py", line 60
        sys.stderr.write(f"ERROR: {exc}")
                                       ^

    SyntaxError: invalid syntax