Switch to pyproject.toml to support python 3.12; update default to python 3.10 and drop 3.9; fixes #49

This MR switches to use pyproject.toml instead of setup.py to define package variables. This allows us to drop pkg_resources and setuptools which are depreciated in python 3.12. This type of setup is also considered more modern and easily maintained in comparison. Note python 3.12 tests are still failing due to running out of memory, but no longer due to packaging.

This MR also makes python 3.10 the default version and drops python 3.9 entirely.