Move tox.ini settings to pyproject.toml
Looks like tox will support loading its settings from pyproject.toml in its next version:
- pr: https://github.com/tox-dev/tox/pull/3353
- documentation preview: https://tox--3353.org.readthedocs.build/en/3353/config.html#tox-toml
(following the centralization effort started in !43 (merged))