mpiexec hangs with multiprocessing with a non-fork start method

[mihaic]

See the following example:

mpiexec -n 2 python3 test_pool.py  # runs forever, kill it manually
cat test_pool.py

import multiprocessing

from mpi4py import MPI

def test_pool():
    multiprocessing.set_start_method("spawn")
    with multiprocessing.Pool(2) as pool:
        pool.apply(f, ())

def f():
    pass

if __name__ == "__main__":
    test_pool()

This affects us when running on macOS, where the the start method is "spawn". I will disable the affected tests so CI can pass.

We should probably disable multiprocessing in this situation.