This version modernizes the build system and fixes an issue where installation from a source distribution would fail.
Summary of changes:
* Most setup options are now defined in `setup.cfg`.
* The `requirements.txt` file has been removed.
* The `upload.sh` script has been removed; uploading to PyPI is handled by a GitHub Workflow.
* Extensions are now either built from a `.pyx` file if available, or a `.c` file otherwise.