Cannot install without Cython being installed beforehand

[k-collie]

This is because the setup.py imports Cython.

PEP518 specifies how packages should communicate such requirements.

Resources

• https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#basic-setup-py
• https://packaging.python.org/en/latest/guides/modernize-setup-py-project/