FIX-#425: Fix type of shared MPI.buffer (#426) #863

Job	Run time
lint (black)	10s
lint (flake8)	4s
test-ubuntu (backend ray, python 3.9)	1m 53s
test-ubuntu (backend mpi, python 3.9)	2m 7s
test-ubuntu (backend dask, python 3.9)	2m 10s
test-ubuntu (backend pymp, python 3.9)	1m 50s
test-ubuntu (backend pyseq, python 3.9)	1m 49s
test-ubuntu (backend ray, python 3.10)	2m 5s
test-ubuntu (backend mpi, python 3.10)	1m 58s
test-ubuntu (backend dask, python 3.10)	2m 13s
test-ubuntu (backend pymp, python 3.10)	1m 44s
test-ubuntu (backend pyseq, python 3.10)	1m 44s
test-windows (backend ray, python 3.9)	6m 5s
test-windows (backend mpi, python 3.9)	5m 5s
test-windows (backend dask, python 3.9)	5m 14s
test-windows (backend pymp, python 3.9)	4m 48s
test-windows (backend pyseq, python 3.9)	4m 37s
test-windows (backend ray, python 3.10)	4m 53s
test-windows (backend mpi, python 3.10)	4m 39s
test-windows (backend dask, python 3.10)	4m 18s
test-windows (backend pymp, python 3.10)	4m 15s
test-windows (backend pyseq, python 3.10)	5m 34s
	1h 9m 15s

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