Cython dependency for downstream recipes

[llperez]

• I read the conda-forge documentation and could not find the solution for my problem there.

Issue:

I'm trying to add a feedstock recipe for a package that depends on hdbscan: conda-forge/staged-recipes#18064
The checks won't pass unless cython is declared as a dependency--the logs explicitly state that hdbscan requires it, even though the package I'm adding doesn't.