Add six module into openstack-cleanup/requirements.txt (#10099)

To fix tf-elastx_cleanup job which was failed with the following error:

   File "/usr/local/lib/python3.11/site-packages/keystoneauth1/identity/generic/password.py", line 16, in <module>
     from keystoneauth1.identity import v3
   File "/usr/local/lib/python3.11/site-packages/keystoneauth1/identity/v3/__init__.py", line 27, in <module>
     from keystoneauth1.identity.v3.oauth2_mtls_client_credential import *  # noqa
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/keystoneauth1/identity/v3/oauth2_mtls_client_credential.py", line 17, in <module>
     import six
 ModuleNotFoundError: No module named 'six'