Major update:
- Overhauled packaging to modern PEP517 standard (with build isolation)
- Added continuous integration with GitHub Actions (which stopped working since Travis stopped offering a free tier)
- Cythonization must now be explicitly specified (instead of being done by default), using the
--cythonizeflag (please see the README). - Python 2.7 and <= 3.6 dropped, but compatibility up to 3.11 is now ensured.