title: MPI4PY OverflowError: integer 2333441511 does not fit in 'int'
body: <p>I scatter list of nested dict without any number bigger than 200000, all the size of which are about 500M.</p>
  
  <pre><code>Traceback (most recent call last):
    File "test.py", line 220, in &lt;module&gt;
      recv = comm.scatter(send, root=0)
    File "MPI/Comm.pyx", line 1286, in mpi4py.MPI.Comm.scatter (src/mpi4py.MPI.c:109079)
    File "MPI/msgpickle.pxi", line 707, in mpi4py.MPI.PyMPI_scatter (src/mpi4py.MPI.c:48114)
    File "MPI/msgpickle.pxi", line 168, in mpi4py.MPI.Pickle.dumpv (src/mpi4py.MPI.c:41672)
    File "MPI/msgbuffer.pxi", line 35, in mpi4py.MPI.downcast (src/mpi4py.MPI.c:29070)
  OverflowError: integer 2333441511 does not fit in 'int'
  </code></pre>