modernize project config, move to pyproject.toml

[skshetry]

• Moves config from setup.cfg to pyproject.toml
• Removes tox.ini as it does not seem to be used anywhere.
• Moves pytest config from pytest.ini to pyproject.toml.
• Moves flake8 config to .flake8 - it does not support config in pyproject.toml (maybe worth moving to ruff?).

pyproject.toml was generated using pdm import setup.py command.

[shcheklein]

thanks @skshetry !