Add tox workflow for Python CLI linting

[andmat900]

Applicable Issues

Fixes #495

Description of the Change

Add a GitHub Actions workflow that runs tox on the Python CLI code in the cli/ directory. This runs black, pylint, and pydocstyle checks on push to main and on pull requests, following the same pattern used in other ETOS repositories (e.g. etos-api, etos-library).

Also updates cli/tox.ini to remove the unused py3 test environment (no tests exist) and extends pylint and pydocstyle to cover all Python packages (etosctl and etos_debug in addition to etos_client).

Alternate Designs

Could have added tox as a job in the existing lint.yml workflow, but a separate workflow file is consistent with how other ETOS repositories handle Python linting.

Possible Drawbacks

The newly added etosctl and etos_debug packages may have existing lint issues that will now be flagged.

Sign-off

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Signed-off-by: Andrei Matveyeu, andreimu@axis.com