[FEAT] Add tox-based testing environment for OpenStack users

[paikend]

Summary

Add tox configuration following OpenStack standards while keeping uv for development.

Motivation

Most OpenStack projects use tox for testing and CI/CD. Our project should follow these standards to be familiar to OpenStack contributors.

Use cases:

• Run tests: tox -e py3 or tox -e py310
• Run linting: tox -e pep8
• Generate coverage: tox -e cover
• Build docs: tox -e docs
• Run in venv: tox -e venv -- <command>

Proposal

Add tox.ini following OpenStack SDK patterns:

• Basic environments: py3, py{310,311,312}
• Linting: pep8 (using ruff instead of flake8)
• Coverage: cover
• Venv: venv for arbitrary commands
• Consider using tox-uv plugin for performance while keeping OpenStack-style configuration

Keep existing uv workflow for fast local development.

Additional context

Reference: openstacksdk tox.ini

Related to #76 - Supporting various package managers for common OpenStack users.