mpi4py-fft cannot be cross built from source because its sphinx
dependency is not satisfiable. The package already separates its
documentation to an arch:all package and limits the documentation build
to the indep part. The only reason to keep the sphinx dependency is for
using the debhelper addon. When using debhelper addons declaratively
(Build-Depends: dh-sequence-*), they can also be moved to
Build-Depends-Indep and then python3-sphinx can also be moved there. I'm
attaching a patch for your convenience and used reproducible builds to
verify that it does not introduce unwanted changes.

Helmut