- Remove Python 3.7 from the test matrix and list of supported versions
  in `setup.py`, as it reached end of life on 2023-06-27
- Add Python 3.12 to the list of tested / supported versions as the
  current major version

- Replace usage of `python setup.py test` and the `PyTest` test command
  with a more direct invocation of `pytest` in `tox.ini`
- Replace usage of `pkg_resources.get_distribution` with
  `importlib.metadata` to get the installed version number for
  `vladiate`