moving mpi4py to Debian Science

[Drew Parsons]

mpi4py has been lagging badly behind upstream. The Debian version hasn't 
been updated since 2017 with 2.0.0-3.  Upstream is now up to 3.0.2.

I propose moving mpi4py into the Debian Science team. It will fit there 
alongside mpich, h5py, petsc4py, etc.

If there is any reason for keeping mpi4py at version 2.0.0 then please 
let me know.

Drew