title: devstack fails with error markers 'python_version == "3.4"' don't match your environment
body: <p>I've created a virtualbox image with ubuntu 16.04 LTS desktop version and followed the steps with a simple minimal local.conf: <a href="https://docs.openstack.org/devstack/latest/index.html" rel="nofollow noreferrer">https://docs.openstack.org/devstack/latest/index.html</a></p>
  
  <p>The problem is that <code>stack.sh</code> fails with this output:</p>
  
  <pre><code>+ inc/python:pip_install:359:   sudo -H http_proxy= https_proxy= no_proxy= PIP_FIND_LINKS= SETUPTOOLS_SYS_PATH_TECHNIQUE=rewrite /usr/local/bin/pip2.7 install -c /opt/stack/requirements/upper-constraints.txt systemd-python
  Ignoring asyncio: markers 'python_version == "3.4"' don't match your environment
  Ignoring asyncio: markers 'python_version == "3.5"' don't match your environment
  Ignoring dnspython3: markers 'python_version == "3.4"' don't match your environment
  Ignoring dnspython3: markers 'python_version == "3.5"' don't match your environment
  Ignoring mypy: markers 'python_version == "3.4"' don't match your environment
  Ignoring mypy: markers 'python_version == "3.5"' don't match your environment
  Ignoring jeepney: markers 'python_version == "3.4"' don't match your environment
  Ignoring jeepney: markers 'python_version == "3.5"' don't match your environment
  Ignoring SecretStorage: markers 'python_version == "3.4"' don't match your environment
  Ignoring SecretStorage: markers 'python_version == "3.5"' don't match your environment
  Ignoring Django: markers 'python_version == "3.4"' don't match your environment
  Ignoring Django: markers 'python_version == "3.5"' don't match your environment
  Ignoring cmd2: markers 'python_version == "3.4"' don't match your environment
  Ignoring cmd2: markers 'python_version == "3.5"' don't match your environment
  Ignoring typed-ast: markers 'python_version == "3.4"' don't match your environment
  Ignoring typed-ast: markers 'python_version == "3.5"' don't match your environment
  Requirement already satisfied: systemd-python===234 in /usr/local/lib/python2.7/dist-packages (from -c /opt/stack/requirements/upper-constraints.txt (line 417))
  You are using pip version 9.0.3, however version 18.0 is available.
  You should consider upgrading via the 'pip install --upgrade pip' command.
  </code></pre>
  
  <p>I've already upgraded the pip to version 18.0 with <code>sudo pip install --upgrade pip</code> from <code>stack</code> user and executed again the <code>stack.sh</code> script and fails again with same error:</p>
  
  <pre><code>...
  Collecting virtualenv===16.0.0 (from -c /opt/stack/requirements/upper-constraints.txt (line 532))
    Using cached https://files.pythonhosted.org/packages/b6/30/96a02b2287098b23b875bc8c2f58071c35d2efe84f747b64d523721dc2b5/virtualenv-16.0.0-py2.py3-none-any.whl
  Installing collected packages: virtualenv
    Found existing installation: virtualenv 16.0.0
      Uninstalling virtualenv-16.0.0:
        Successfully uninstalled virtualenv-16.0.0
  Successfully installed virtualenv-16.0.0
  You are using pip version 9.0.3, however version 18.0 is available.
  You should consider upgrading via the 'pip install --upgrade pip' command.
  </code></pre>
  
  <p>I've upgraded pip with the command above and it has displayed success, but after running again <code>stack.sh</code> the version is back again:</p>
  
  <pre><code>pip -V
  pip 9.0.3 from /usr/local/lib/python2.7/dist-packages (python 2.7)
  </code></pre>
  
  <p>Then I've upgraded again with: <code>sudo easy_install -U pip</code>
  and it displayed successfully upgraded and finally I've tried with this command: <code>pip install --upgrade --force-reinstall pip</code></p>
  
  <p>And <code>pip -V</code> displays after reinstall:</p>
  
  <p><code>pip 18.0 from /usr/local/lib/python2.7/dist-packages/pip (python 2.7)</code></p>
  
  <p>but <code>stack.sh</code> still fails with the same output error. 
  Then it seems like stack.sh downgrade pip for some reasons.</p>