Missing requirement 'six' which was added in 0de8b41768

[Niels Huylebroeck]

Steps to reproduce

```
# virtualenv six_missing
# source six_missiong/bin/activate
# pip install python-dracclient
# python
Python 3.7.4 (default, Jul 16 2019, 07:12:58)
[GCC 9.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from dracclient import client
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/id856320/t/lib/python3.7/site-packages/dracclient/client.py", line 24, in <module>
    from dracclient.resources import bios
  File "/home/id856320/t/lib/python3.7/site-packages/dracclient/resources/bios.py", line 20, in <module>
    from dracclient.resources import lifecycle_controller
  File "/home/id856320/t/lib/python3.7/site-packages/dracclient/resources/lifecycle_controller.py", line 16, in <module>
    from dracclient import utils
  File "/home/id856320/t/lib/python3.7/site-packages/dracclient/utils.py", line 22, in <module>
    from dracclient import wsman
  File "/home/id856320/t/lib/python3.7/site-packages/dracclient/wsman.py", line 16, in <module>
    import six
ModuleNotFoundError: No module named 'six'
>>>
```

In commit a dependency on `six` was added without it being listed in `requirements.txt`
https://opendev.org/openstack/python-dracclient/commit/0de8b417683527132f7ae6bc9a403a5a83b99488