Outdated - OpenStack command-line clients Installation Update Request

[cgcgcgcgcg30]

- [ ] Update cli installation documentation to reflect Python 3.0 support in >=python-openstackclient-6.0.0 versions.

```
pip install https://files.pythonhosted.org/packages/e7/7c/12bcf0aa9f564e5df663df93dacae6cbd67151df17c55f5f7d9640381a52/python-openstackclient-6.0.0.tar.gz#sha256=91c3ac12da4b423c16b3917616f84a23e2961e750fce590d409ecc50ee0431ce
WARNING: pip is being invoked by an old script wrapper. This will fail in a future version of pip.
Please see https://github.com/pypa/pip/issues/5599 for advice on fixing the underlying issue.
To avoid this problem you can invoke Python with '-m pip' instead of running pip directly.
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
Defaulting to user installation because normal site-packages is not writeable
Collecting https://files.pythonhosted.org/packages/e7/7c/12bcf0aa9f564e5df663df93dacae6cbd67151df17c55f5f7d9640381a52/python-openstackclient-6.0.0.tar.gz#sha256=91c3ac12da4b423c16b3917616f84a23e2961e750fce590d409ecc50ee0431ce
  Downloading python-openstackclient-6.0.0.tar.gz (838 kB)
     |████████████████████████████████| 838 kB 684 kB/s
ERROR: Package 'python-openstackclient' requires a different Python: 2.7.18 not in '>=3.8'

```

-----------------------------------
Release: 1.0.0 on 2019-08-23 19:37
SHA: f80f22b2537844fc48e4f78bfb9eb664ad1f1280
Source: http://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/user-guide/source/common/cli-install-openstack-command-line-clients.rst
URL: https://docs.openstack.org/newton/user-guide/common/cli-install-openstack-command-line-clients.html