Docs on Cython extensions and install requirements

[jakirkham]

What is your question?

It appears that the documentation around Cython extensions includes Cython in install_requires. While Cython is a dependency in this case, it usually doesn't make sense to have it install_requires (particularly if users are merely using Cython to build their package). In this case, it may make more sense to include pyproject.toml. If it is being used to compile code at runtime, this is a different story. Is the intent for users to have compilation occur at runtime using Cython or is it to build extensions that are later used?