Related to #651

On Debian 12, I didn't manage to build mpi4py 4.0.3 because of a very long build (more than 40 minutes) with very high memory consumption (more than 12 GB).

In contrast mpi4py 3.1.6 can be built in few seconds.

gcc (Debian 12.2.0-14+deb12u1) 12.2.0

It might be related to a new Cython version, for example Cython 3.1 was released on May 2025 (https://pypi.org/project/Cython/#history).

In mpi4py 3.1.6, there is setup_args['setup_requires'] += ['Cython>='+CYTHON+',<3.0.0'] while this line is no longer present for mpi4py 4 (but cython calls seem to be done through Meson).