Remove pytest-runner, use SPDX license, use PEP 517

Tests are bugged, and upstream has 3 new commits (yet unreleased, though) to fix tests and drop deprecated imp module. My fix is only to actually allow Python 3.12 rebuild at some point and run the tests, because running python setup.py pytest will fail with error: invalid command 'pytest'.

TODOs related: pytest-runner removal, stop using python setup.py test, imp module removal in Python 3.12