Specify build backend and move project metadata to pyproject.toml

[mwtoews]

This PR does two main things:

• Specify a package build backend, based on setuptools and Cython (PEP 517 and PEP 518)
• Move most of the static project metadata from setup.py to pyproject.toml (PEP 621), as supported by setuptools

Other minor changes:

• Add README.rst to package information (currently it shows "The author of this package has not provided a project description")
• Add two URLs: Homepage and Repository
• Use pip install . instead of directly invoking setup.py (see Why you shouldn't invoke setup.py directly)
• Run twine check --strict before upload to ensure packages looks good
• Add a "test" extra install to do (e.g.): pip install -e .[test] && pytest

[mwtoews]

The last commit does the following:

• Set Cython language_level=3 and regenerate core.c with the latest stable Cython version
• Re-add cp39-macosx*, which appears to build OK now
• Add cp311 to get Python 3.11 wheels
• Skip *musllinux* since NumPy does not build these binary wheels