FIX-#425: Fix type of shared MPI.buffer #861

Job	Run time
lint (flake8)	6s
lint (black)	5s
test-ubuntu (backend ray, python 3.9)	2m 4s
test-ubuntu (backend mpi, python 3.9)	2m 15s
test-ubuntu (backend dask, python 3.9)	2m 5s
test-ubuntu (backend pymp, python 3.9)	1m 41s
test-ubuntu (backend pyseq, python 3.9)	2m 8s
test-ubuntu (backend ray, python 3.10)	2m 3s
test-ubuntu (backend mpi, python 3.10)	2m 35s
test-ubuntu (backend dask, python 3.10)	2m 10s
test-ubuntu (backend pymp, python 3.10)	1m 53s
test-ubuntu (backend pyseq, python 3.10)	1m 50s
test-windows (backend ray, python 3.9)	5m 10s
test-windows (backend mpi, python 3.9)	5m 21s
test-windows (backend dask, python 3.9)	4m 47s
test-windows (backend pymp, python 3.9)	4m 44s
test-windows (backend pyseq, python 3.9)	5m 20s
test-windows (backend ray, python 3.10)	4m 44s
test-windows (backend mpi, python 3.10)	5m 10s
test-windows (backend dask, python 3.10)	5m 53s
test-windows (backend pymp, python 3.10)	3m 52s
test-windows (backend pyseq, python 3.10)	3m 51s
	1h 9m 47s

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