Comment 1 for bug 1948554

The thing is, Cython isn't required, unless you build from a clean source checkout. The source distribution of a normal release does not have any Python dependencies at all, it only needs Python and a C compiler.

I guess this could be expressed with the build-sdist hook:
https://www.python.org/dev/peps/pep-0517/#get-requires-for-build-sdist

To me, it doesn't really seem worth the hassle of setting up a local build backend just for this case. Can't say if others feel differently about this.