Integration of various commands via setuptools is deprecated or is
discouraged, so we are moving that to tox.

Namely pytest and flake8.

It will also resolve following things:
* Testing multiple python versions in sane manner. Use python 3.9 as
  default interpreter for now (TOS 6.0+).
* flake8 setuptools integration
* Dependency lookup issues with latest pip (v23.x) and setuptools (v67.x).
* Dropping obsolete setuptools keywords (tests_require, setup_requires,
  dependency_links, ...) from setup.py.