Update to the latest recommendations for using setuptools as the build
backend [1].

The setup.py script is removed. SimPy packages must now be built using
the recommended `build` frontend tool (i.e. `python -m build`).

The project configuration is moved from setup.cfg to pyproject.toml [2].

The version, which is derived from git by the setuptools_scm package, is
now interrogated at runtime by importlib.metadata instead of
pkg_resources. This is the contemporary recommendation of setuptools_scm
[3] and is needed for Python 3.12 where setuptools and pkg_resources are
no longer available by default in runtime environments [4]. The
importlib.metadata mechanism is available as of Python 3.8, which is
conveniently SimPy's new minimum required Python version.

[1]: https://setuptools.pypa.io/en/latest/userguide/index.html
[2]: https://peps.python.org/pep-0621/
[3]: https://setuptools-scm.readthedocs.io/en/latest/usage/
[4]: https://peps.python.org/pep-0632/