Fixed tox python version matrix

[dhruvDev23]

Updates

• Changed tox test matrix to match requires-python in pyproject.toml.
• Removed references to unsupported Python versions (3.7, 3.8).
• Dropped setup.py from flake8 checks since it does not exist.
• Updated [gh].

[Copilot]

Pull request overview

This PR updates the tox configuration to align with the project's supported Python versions as defined in pyproject.toml, removing support for Python 3.7 and 3.8 while adding support for Python 3.9 through 3.13.

• Updated tox test matrix from Python 3.7-3.8 to Python 3.9-3.13
• Modernized the build command from setup.py to python -m build
• Removed non-existent setup.py from flake8 checks

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[marco-2023]

Thanks @dhruvDev23 for your contribution. Unfortunately, we decided to drop tox for grid in #300 to improve package maintainability. This PR is no longer up to date.