Python version updates

[JoFrhwld]

The latest supported version listed on pypi is 3.9, which is approaching end of life. Updated the supported versions was a but more complex, as distutils was removed from v3.12.

PR Explanation

I did:

• I converted the metadata from setup.py and setup.cfg to pyproject.toml. This was all "by hand" to keep dependencies minimal, so no additional build tools are necessary beyond setuptools.
• I updated the github action to test on 3.9 through 3.13, as well as the versions the checkout and python setup actions.
• To maintain dynamic versioning, like there was with setup.py, I added a __version__ attribute to pympi/__init__.py. When the package is built, this value will be dynamically used.

I did not:

• I did not bump the version, so it is still v1.70.2 as it is in the main branch.

Working locally:

To build the package:

python -m build

Testing still works as before with

pytest