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How to build a tox multi-env matrix with mock config in Python

Simulate a tox multi-environment matrix by validating environment names and grouping extras into a readable matrix structure.

tox ci matrix
Python 
```python
import tox

def run_tox_matrix(mock_envs):
    """Simulate a tox multi-env configuration and verify mock choices."""
    config = {
        "tox": {
            "envlist": mock_envs,
            "config": {
                "basepython": "python3.9",
                "deps": ["pytest", "mock"],
            },
…
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