support for tox into setup.cfg

Tox can be configured also into setup.cfg file, e.g.

```
[tox:tox]
isolated_build = true
envlist = lint, format, py3.8, py3.9, py3.10, py3.11, build
```

or, using default naming convention
```
[tox:tox]
isolated_build = true
envlist = lint, format, py38, py39, py310, py311, build
```

No source file or test is named in the issue. Start by locating the existing tox.ini configuration handling and the setup.cfg parsing path; use the two [tox:tox] examples as input cases and confirm both envlist naming styles are recognized.

Written by the indexing model from the issue text.