Support for Python 3.12 and PyPy 3.10

[movermeyer]

What are you trying to accomplish?

Let's get CI working with the latest version of Python.

What approach did you choose and why?

• Added Python 3.12 everywhere, and PyPy 3.10 to CI
• Migrated our tox configuration to the v4 syntax, as per their migration guide

What should reviewers focus on?

🤷‍♂️. There's a new deprecation warning related to our use of the antiquated python setup.py test:

$ python setup.py test
running test
WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.
/home/mike/.pyenv/versions/3.12.0/lib/python3.12/site-packages/setuptools/command/test.py:193: _DeprecatedInstaller: setuptools.installer and fetch_build_eggs are deprecated.
!!

        ********************************************************************************
        Requirements should be satisfied by a PEP 517 installer.
        If you are using pip, you can try `pip install --use-pep517`.
        ********************************************************************************

!!
  ir_d = dist.fetch_build_eggs(dist.install_requires)
WARNING: The wheel package is not available.

But getting a PEP 517-compliant builds and tests working in a way that supports all the old versions of Python that we support seems like a lot of work, so that'll be done in a separate issue.

The impact of these changes

We'll be able to give confidence that ciso8601 is still working on modern Python.