Update tox.ini and pyproject.toml to complete Python 3.8 deprecation.

[Strift]

Description

Following the deprecation of Python 3.8, we need to update the following:

1. tox.ini line 2: Remove py38 from envlist—currently reads envlist = pylint, mypy, py38, py39, py310, py311 but should drop py38
2. pyproject.toml [tool.black]: Update target-version = ['py38'] to target-version = ['py39'] (or ['py39', 'py310', 'py311', 'py312', 'py313']) to align with requires-python declaration

These configurations directly contradict the dropped Python 3.8 support and will cause issues in the CI/local test environments.