Cleanup the tox configuration and requirements.
Test Plan:
Verify that all Tox tests pass without errors.
1. Successfully executed on the distributedcloud dir
- tox -e docs,releasenotes,api-ref,bandit,linters
- tox -e newnote -- new-note
2. Successfully executed on the distributedcloud/distributedcloud dir
- tox -e py39,pylint,pep8
Partial-Bug: 2033294
Change-Id: Ie7b2acb03c379708e8ef57b52d3c83560414f007
Signed-off-by: Hugo Brito <hugo.brito@windriver.com>