[Python] current python package require system mpi

Problem

Currently mpi4py is included in the python requirements in dune-common. On systems with MPI there were occasional problems with running python scripts where MPI was initialized on the C++ side. Instead we have

try:
  from mpi4py import MPI
except ImportError
  pass

in dune.common.__init__.py to avoid this issue. On system without MPI pip install dune-common fails since mpi4py cannot be build.

Solution

No idea how one can have optional dependencies in setup.py