This covers the following updates to fix CI currently broken.
* Fix compatibility with tox 4
* Update hacking to 6.1.0
* Clean up python 2 support and bump minimum supported version to 3.8
* Remove six because python 2 support is removed
* Update job template to use the recent tested python versions
* Replace items by prefixItems to fix schema error
* Build documentation by sphinx command
* Remove "Change Log" section from documentation
* Split requirements for documentation build
* Ensure tox is installed in functional tests
* Fix devstack job
Change-Id: I3b9c5b20aca55332c721d34fd4c41c5b886f60c5
